THERES A BIT OF SURFACE CURRENT, warns the skipper as we are queuing to jump off the dive boat, So dont hang around.
     After a prolonged wait in the heat of a Tunisian afternoon, it is sheer relief to hit the water. I grab at the large orange buoy, dump all the gas from my wing, and exhale through my mask. In fabulous visibility I can see the line stretching away into the blue beneath me, but Im not dropping.
     The skipper wasnt joking about the current. Im strung out like a flag in the breeze and the only way down is to climb, hand over hand, my legs flying out behind me.
     Beyond 40m, the pull begins to ease and I shudder as a thermocline reduces the water temperature from a cosy 26C to a chilly 19. At this moment my two skimpy layers of wetsuit dont seem quite such a bright idea.
     The sea around me is a rich delphinium blue, but at 50m there is still no sign of the wreck below. As the depths turn brown and the temperature drops to 16, the upturned edge of the deck comes into focus. The ship is lying on her starboard side, with her deck at a steep, almost 90 angle.
     Now I realise that the depths are not brown. The wreck is brown, and it stretches away beyond the limits of the visibility. HMS Manchester is a huge 9500 ton warship and here I am, a tiny yellow speck, scattering the shoals of silver reef fish as I drop from the line onto three prongs rising up to meet me.
     It takes me a moment to settle, check that everything is OK and turn on my torch. In the gloom below I can see the lights of divers exploring the lower reaches of the wreck, but Im looking up.
     These three prongs, encrusted in marine growth, are 6in guns. Im sitting on a gun turret at 75m, transfixed by weapons that were fired 60 years ago, shortly before a torpedo ripped into the aft engine killing 15 men and rendering the ship powerless.

The sinking of HMS Manchester
It was the most frightening moment of my life when that torpedo hit, Eddie confides when Im back on the boat. The entire ship shuddered. It was awful.
     Eddie Pykett (Stoker First Class) and Allan Walker (Leading Steward) are veterans from HMS Manchester. Both in their 80s, they have travelled to Tunisia to join the expedition aboard Princess Duda, a Maltese-based liveaboard, to observe the diving and share the discoveries from the video footage brought back after each dive.
     The last time that they stood on the deck of the Manchester it was night, and they were being told to abandon ship and make for the faint lights along the coast of Tunisia.
     HMS Manchester was part of the largest naval convoy in history, Operation Pedestal, tasked with protecting 14 merchant ships attempting to supply the besieged island of Malta in August 1942.
     After surviving three days of relentless air attacks, a fast-moving Italian E-boat torpedoed the cruiser at night, hitting the aft engine. Captain Harold Drew had 400 men, including all the injured, taken off by an Allied ship before telling the remaining 500 crew to abandon ship, setting scuttling charges to ensure that it sank rather than fell into enemy hands.
     He had no choice, maintains Allan, with a distant look in his eye.
     All of the ships engines were out of action, we were drifting and we had no power - we couldnt fire the guns. The ship was sinking.
     Dawn would bring more air strikes and HMS Manchester was a sitting duck. I remember him saying: ÔYour lives are now more valuable than the ship, recalls Eddie, his admiration for the captain still apparent.
     The decision to abandon ship was one that the Navy rewarded with a court martial, to the crews dismay.
     The job of the dive team and film crew is to explore the wreck and record the extent of the damage. Perhaps when faced with factual evidence about the situation faced by Captain Drew, the blight on his reputation can be erased.
     That is the hope of Simon Bennett, the expedition leader, and Crispin Sadler, producer of the TV documentary about HMS Manchester and Operation Pedestal for Carlton TV.

Encountering the break
I drop through the water with a memorised picture of the ships plans to pinpoint where our shot is lying. The sheer size of the wreck and the number of different types and sizes of guns is hugely distracting. I pause for a second and try to find a non-existent suit inflator on the front of my wetsuit. Get a grip!
     I can hear my buddy chuckling beside me, so I motion that we should go forward. Im trying to avoid stirring up the thin layer of silt that has gathered on the deck, so that the video footage will capture the wreck clearly.
     A gaping black hole close to the seabed takes me by surprise, and I pause to figure it out. The edges are far too even to have been caused by an explosion. Looking closely, I can see the faint profile of a cog. This is a gun turret, but the mounting containing the battery of three guns has fallen out. Inside I can make out a shell casing and, on the sand gathered at the bottom of the hole, a lantern with the glass still intact.
     As I swim further, the wreck appears to come to an end, the hull collapsed onto the seabed. There is a massive break, in excess of 10m - I cant see across it to where I know the rest of the wreck must be.
     An enormous section midships has been punched out of the ship. This is the area above the engines, where the torpedo hit. I swim up onto the top railing. Surveying the damage from here, it seems amazing that the ship could have stayed in one piece after such an impact.
     Unfortunately, while Captain Drew and most of his crew were held in a POW camp by the Vichy French, some of the officers he had sent to safety with the wounded men were voicing the opinion that HMS Manchester might have been able to make it to a neutral port.
     Even taking into account the damage caused as the ship sank and impacted with the seabed, and some damage that would have been caused by the scuttling charges set in the bow, HMS Manchester had clearly been gutted like a fish by the torpedo hit. From what I saw, perched on the upturned deck and looking out across the wreck, the only place that that ship could possibly have gone was down.
     Back aboard, Canadian divers David Sawatzky and Ralph Hoskins tell me that they have swum across the collapsed area of debris and onto the bridge. They have seen the full extent of the damage and it includes all four engines. The veterans were not exaggerating when they recalled that the ship was powerless. Surely, now, the decision to court- martial Captain Drew can be challenged
     Allan, a Navy man his entire life, appears less than optimistic about the authorities admitting to a mistake. It was made clear to Captain Drews wife before the war that he had enemies in the Admiralty, he tells me with a weary, knowing smile. I get a feeling that encountering the break might prove easier than uncovering the real reasons behind such an obvious injustice.
     Captain Drew was never given another command at sea. He died in 1987 and his daughter says he never quite got over the stain on his character. The surviving crew in the HMS Manchester Association never quite got over it either. With the help of the dive team, they will not let the matter lie.

The diving operations
The strangeness of the diving procedures intrigues the two veterans. They bring chairs out on deck and watch each teams elaborate ritual of kit-preparation and suit-donning, discussions of run-times, clipping on of stages, and the endless hosing down while we wait for the signal to move to the exit and the OK to jump. All this is necessary to get to 80m and back in safety.
     For the 20 or 25 minutes that I spend exploring the wreck, I have to complete at least an hour of decompression. Ascending from the wreck feels fantastic. I watch the dark bulk of the structure becoming an indistinct blur below me as I move up the line into increasingly warm water. On the wreck, the excitement of the dive makes it is easy to forget about the chill, but once the warmth returns, I begin to appreciate how cold I must have been.
     Dive Marshal Sharron Rendell is my hero. When I jump off the boat, my only concern is to do the dive, keeping myself and my buddy safe. Sharron has two deco stations to deploy, and 20 divers (including a film crew) to worry about, split into four teams and all doing 80m dives. Whatever mistakes we might make - missing the shotline, surfacing prematurely, running low on gas, coming back with a bend or other injury - she must be ready to deal with.
     From the first diver going in to the last one returning takes roughly four and a half hours. For every minute Sharron is working, watching and worrying.
     Her watchfulness is shared by Jeannie, who spends the entire period of the diving operations on the bow of Princess Duda in the fierce midday heat with a pair of binoculars. If a diver were to put up an SMB or surface unexpectedly - situations that would indicate some kind of problem - Jeannie is there to raise the alarm.
     Dont you get really fed up with it I ask her after one dive. No. Thats my man down there, she replies simply.

Missing props and missing the boat
Today the current is negligible, and it is no effort at all to slide down the line, pausing briefly at 30m to clip my tag onto the end of the lazy shotline that leads to the deco station. My buddy Mark is determined to visit and film the props. Unfortunately someone else has got there first.
     The MoD sold the salvage rights in the 1950s and the props have been removed. Its a sorry sight to see two stunted propshafts, encrusted with marine growth, but he films the severed ends anyway.
     Back on the boat, we discuss the prop situation with the veterans before showing them the video. I look over at Allan, concerned that he will be angry or upset. After all, several of the crew were killed in the torpedo attack and the ship is a war grave. They wouldnt have minded about that, he says, shaking his head and smiling. The ones down there, it wouldnt have bothered them. And he gives a small dismissive wave of his hand.
     So what happened to you when the ship went down I ask Eddie. It seems that after the engines had been put out of action, the captain called everyone on deck and told them to abandon ship.
     Everybody else took the small wooden rowing boats, but Eddie volunteered to set scuttling charges in the bow, and returned to deck to find all the boats gone.
     Did you have a lifejacket I ask.
     No, Id left it below, he explains with a shrug. They werent much good anyway.
     I try to imagine how it must have felt to be left on the deck of a sinking ship, at night, miles from shore. So you swam
     Thats right, he says. I just headed for the lights. When I got tired I lay on my back and rested for a while.
     Allan had been lucky and got a ride in the engine-driven support boat with the captain, but it took them four hours to reach shore. I ask Eddie how long it took him to swim, but he has no idea.
     When I got to the beach, l just fell asleep, like everybody else. We were all exhausted. Wed all been on duty continuously for three days and nights.
     It hadnt really occurred to me before that in battle, nobody stops for a break.

Memories, insights and souvenirs
The video footage of the wreck is stirring up lots of memories. Allan keeps telling me where to find his quarters. I think he is hoping that some of the plastic-wrapped cigarettes that he had stashed may still be intact. Its on the port side, you go down this corridor and its opposite the mess he says, pointing on the ships plans.
     But were not allowed to enter the wreck or take anything off it. The Tunisian Navy is represented on board to make sure we obey the rules. Id just like a souvenir, he says. Maybe you could bring me a piece of that sponge thats growing on her.
     Eddie seems more interested in the fate of the Navy rum. It was in oak caskets - Im sure theyre watertight, he tells me hopefully. And we laugh about the possibility of sneaking a barrel of rum off the wreck and back onto the dive boat.
     The next day, positioned above the wreck, a service of remembrance for HMS Manchester is held and the veterans put on their medals and uniforms and hold wreaths. Simon Bennett is reading a tribute to those lost in Operation Pedestal and on the Manchester. Everybody else retreats behind the TV cameraman to avoid cluttering the shot.
     The veterans are composed and professional in the fierce heat, but Simons voice is choked with emotion. For Simon, this is not just another dive, and whatever ideas and preconceptions the rest of us had have been transformed by diving this wreck in the presence of the veterans.
     What was the POW camp like I ask Allan. The veterans are recounting how, after they reached the beach in Tunisia, they were rounded up by the Vichy French, who controlled Tunisia at the time.
     We were transported to Algiers, and taken to a camp right in the middle of the desert. They didnt treat us badly, but we didnt get much to eat, says Allan with a wry smile. But luckily we were freed after three months.
     The success of Operation Pedestal allowed fuel, food, ammunition and equipment to reach Malta. The forces in Malta were able to prevent Rommels troops in North Africa receiving supplies, enabling the Allies to seize control of the area. The luck is that it came just in time for the Manchester crew.
     I lost lots of weight in the camp. I only weighed 5 stone at the end, Allan tells me, without any trace of self-pity.
     After that, I cant ask any more questions. The image of Eddie and Allan as human skeletons is stuck in my head.
     Today the sea is a bright, friendly blue and the visibility seems even more stunning than usual. The shot has been moved forward, close to the bow. The two Canadian cave-divers Dave and Ralph have done a superb job of tying-in.
     Dropping onto the wreck, I instantly recognise the distinctive rows of windows that line the bridge, now empty of glass.
     To my side I watch the Cheltenham team of Simon, Ash, Ade and Andy unfurling the Royal Navy ensign.
     They pause, and lay the plaque down on the bridge before carefully wrapping it in the flag and leaving it inside the wreck. Mike Pitts, more used to filming dolphins for The Blue Planet, follows their every move with his camera.
     Alighting close to the forward guns, I pluck a clump of yellow sponge fingers from beside the bridge and stow it safely in my pouch as a souvenir for Allan. Its a small gesture, but in my own way, I feel that Ive completed a mission.

UK: Simon Bennett (project organiser), Jean Bowers (surface support), Mark Brill, Ashley Gibbons, Paul Harrington, Liz McTernan, Andrew Mizzi, Neil Plant, Sharron Rendell (dive marshal), Adrian Street, Louise Trewavas
Canada: Ralph Hoskins, David Sawatzky
Malta: Paul Ciappara, Emi Farrugia, Mario Gauci, Sarah Gauci Carlton
South Africa: Christian Malan
ITV Carlton: Richard Bull, Keith Morris, Mike Pitts (camera), Crispin Sadler (producer)

84-year-old Eddie Pykett, who was a Stoker First Class aboard HMS Manchester, recalls the day his ship went down
Louise Trewavas at 75m on the aft 6in guns
View of the forward gun
on the bridge
Veterans throw wreaths during the service of remembrance
above the fallen gun turret
crane debris amidships