It wasn't the shore leave we expected. But it has changed us all

HMS Chatham was due to dock at Dubai when she was diverted to Sri Lanka as part of the relief effort. Documentary-maker Chris Terrill was on board and describes the change of tack from war to peace

On Christmas Day, HMS Chatham was on patrol in the Arabian Gulf. The 246 men and women on board were making the best of it: there was Christmas dinner, carols on the flight deck and a few presents but a warship is a warship and no amount of tinsel, plastic reindeer and pretend snow can turn it into Santa's grotto. New Year was going to be a different matter. The ship was coming alongside in Dubai - a traditional favourite with the British sailor.

On Christmas Day, HMS Chatham was on patrol in the Arabian Gulf. The 246 men and women on board were making the best of it: there was Christmas dinner, carols on the flight deck and a few presents but a warship is a warship and no amount of tinsel, plastic reindeer and pretend snow can turn it into Santa's grotto. New Year was going to be a different matter. The ship was coming alongside in Dubai - a traditional favourite with the British sailor.

HMS Chatham, a Type-22 frigate, was one of a number of warships policing sea traffic approaching the Persian Gulf in an attempt to stop the smuggling of arms and people into Iraq. It was important work but monotonous so the prospect of "Doobers", as Dubai is known in the Navy, had sustained the crew for weeks.

Nobody had given a second thought to plate tectonics. Well, you don't at Christmas, do you? So nobody was expecting an earthquake of 9 on the Richter scale, followed by a tsunami that devastated a massive swath of the earth's surface causing havoc, destruction and death on the morning of 26 December.

The news shocked everybody on board as it did the entire world. For a time we even braced ourselves in case the tsunami spread into our part of the Arabian Sea. But though its energy was sapped by the deeps well short of our position, it was about to touch our lives in another way.

The night we arrived in Dubai, the captain, Steve Chick, was told to stand by. Next morning, the confirmation came. HMS Chatham would stay in Dubai just long enough to complete some basic maintenance and take on fresh stores, but would sail that evening. Off-duty sailors were allowed ashore but all hands had to be back on board by 1600hrs and ready to sail by 1800hrs. The promised days in Dubai had turned into mere hours.

By 1600hrs on 30 December everyone was on board. Usually, it is said, the British sailor is only happy if he has got something to complain about (it's called "dripping" in the Navy) and foreshortened leave is always guaranteed to get his gander up. But on this occasion there was not a single word from anyone. Everyone had seen graphic news coverage of the death and destruction wrought by the tsunami - the wrecked towns, the razed villages and the shattered lives of millions. On routine patrols in the Gulf it had been difficult to keep a clear perspective of how important the ship's role was because the enemy was invisible and hard to identify. But now, in the face of raw nature and its terrible power, every man and woman in the ship's company was gripped by a grim determination to get back to work.

Surgeon Lieutenant Alison Dewynter, the ship's doctor, was reading up on infectious diseases such as cholera and typhoid. She also downloaded an information sheet from Fleet headquarters: "How to Manage Dead Bodies in Disaster Situations".

"You never know with this job what you are going to be called to do," she told me. "One day it's dealing with a sore throat, the next it's a heart attack and then something like this - but until we get there I don't know what we will be required to do. I just hope we can make some sort of difference."

The Rev Tommy Goodwin, a cheery Scot and the ship's chaplain, was gearing up as well. "The crew are likely to see some terrible things and deal with true tragedy. Some of them are very young and most have never seen a dead body before. This will be a life-changing experience. It will be for me too. I don't know what to expect."

In a remarkable transformation, a lethal, frontline warship started to metamorphose into a benign ship of mercy. The Sea Wolf guided missiles were taken out of their loaders and stowed below; the powerful mini-guns on the port and starboard waists were dismantled as were the machine guns on the forecastle, the bridge wings and the flight deck. The Lynx helicopter machine gun was replaced with a winch. The ops room, the nerve centre of the ship, was reconfigured. It would now be the planning HQ for a major disaster relief operation.

All around the ship, ratings and officers were adapting their conventional roles. Weapons specialists, gunners and stokers might be needed for rebuilding water storage units or reconnecting electricity supplies; radar and sonar operators might have to re-establish telephone lines; chefs and stewards might be drafted for first aid or burial parties.

Three days later we arrived in Sri Lanka. After a briefing from the observation, liaison and reconnaissance team, the ship headed for the east coast, which was the hardest hit by the tsunami. The two Lynx aircraft flew constant reconnaissance sorties and I frequently travelled with them as they circled the devastated areas. We swooped low over countless smashed fishing boats - some had been lifted hundreds of metres inland and rammed into buildings, two or three storeys up. Cars were scattered, overturned, crushed. Huge sections of railway had been ripped up, twisted and contorted. Houses had been reduced to kindling wood. Some of the larger trees left standing were still laden with bodies that had been swept up into their branches.

Looking down on the devastation gave me the impression that the place had been blitzed - systematically destroyed by massive precision bombing that had targeted a three or four hundred metre strip along the entire coastline. Inland of this strip was unscathed but within it, annihilation, in some places, was total. Thousands had perished without warning and yet now, just over a week since the tsunami struck, the heavily silted sea washed gently onto once golden beaches, now littered with the detritus of countless wrecked homes.

Back on the ship everybody was getting impatient to get ashore and do something. But it was not just a simple case of pitching up and getting involved. This was still a military operation and it was being approached as such. There is a saying in the Navy: "Prior planning prevents piss-poor performance." The ship's navigators, hydrographer and boatswains had to find safe landing grounds for the ship's boats - or nobody was going ashore. Small recce parties had to be flown in by helicopter to talk to the local people and any NGOs in the area to decide how best the ship and her company could contribute to the disaster relief. And then there had to be some sort of assessment of priorities. Just where do you start after a tsunami?

We carried on around the south of the island and up the east coast until we got to a place called Batticaloa.

The wave had smashed into this coastal town with such force that many of the estimated 2,000 dead had never been found. Entire neighbourhoods had disappeared. Whole sections of the population were homeless. Wells were full of salt water. And thousands were displaced. Families who for generations had lived by the sea and from the sea were deserting the sea. They had no possessions. They had no livelihood.

Captain Chick and his officers quickly drew up plans. There would be a four pronged attack, as it were, on the needs of the people of Batticaloa and early on the morning of 7 January, work started. A forward party of Royal Marines had already gone in to check the area for landmines. This was a Tamil area and following years of war between the government and the separatist Tamil Tigers, the Sri Lankan army had laid down minefields. Even though there had been a ceasefire for the past two years, the minefields had remained intact - until the tsunami. The giant wave had washed many of the live mines away from their fields and scattered them. Once the marines were happy that the mine threat was minimal, 56 sailors and more marines were landed at beach heads established by the forward recce parties. The waters were treacherous, with many shallows inhabited by saltwater crocodiles.

About 30 sailors went to the northern district, Kallady, and started to clean out any houses that were left standing. In the southern district, work on damaged fishing vessels could not start because nobody could find the owners. Nearby, however, was a Franciscan convent and orphanage called St Theresa's, where the wave had smashed down the perimeter wall. The head of the convent, Sister Irene, asked the sailors if they could help clear away the rubble and put up a fence to give them protection and security. The shore party agreed vigorously and started the work. Huge slabs of concrete and brick were manhandled to the other side of the road, pillars were erected into holes dug at intervals and barbed wire was strung across. The work - backbreaking and sweltering - took all day.

The nuns were, I think, a little non-plussed but grateful nonetheless. Throughout the day they provided fresh fruit and tea. Extraordinarily, on the crockery they brought out was the portrait of a fully bearded British sailor - the one you used to see on the front of a packet of Senior Service cigarettes. They were probably donated to the convent by someone and I do not think the nuns knew that the head on the cups was a portrait of a British sailor and so did not realise the irony. But the sailors did.

I then joined Chief Petty Officer Jim Wharton and Warrant Officer "Simmo" Simpson who were going across the lagoon in a boat to check out a minibus that needed pulling out of the water. It belonged to a farming co-operative and we were accompanied by the co-op secretary, who insisted it was only slightly damaged. Eventually, they found the minibus lying on a sandbank in about four feet of water. It was a hopeless case. The vehicle was upside-down, the chassis was bent, the roof crushed, and the engine by now full of sea water. "Maybe you could fix it," said the optimistic co-op man. "Sorry, mate," said Simmo. "I don't even know a man who could. It's a gonner."

The co-op man looked crestfallen "But I am responsible for this van." "Well, it's been totalled by a tsunami. I don't reckon there's anyone could pin that one on you, mate," said Jim Wharton and the co-op man nodded sadly.

As we walked back to the boat, we found the beach littered with possessions. The arm of a child's doll here, a green and gold sari there, then a black shoe, then a few yards further on a photograph album lying open showing a young couple smiling, then a kettle, another shoe, a lipstick ...

It was then that the co-op man confided he had lost his own mother. Literally. She had been washed out to sea and never found. There was a pause. "Sorry," said Jim Wharton gently and put his hand on the man's shoulder.

When we returned to the convent, Sr Irene was beaming. All the rubble of the old wall had been shifted and a brand-new barbed wire fence stood in its place. During the next few days work continued apace. Diligence, a Royal Fleet Auxiliary support ship, arrived to join the effort and helicopters continually ferried working parties to and fro. As well as house clearance, projects included erecting tents for displaced people, well cleaning and boat repair. One party cleaned out a church so that Mass could be celebrated for the first time since the disaster and another party continued the repairs at St Theresa's convent.

Yesterday, I went with a party to try to reach an orphanage at the end of a peninsula which we had been told needed help. We trekked for hours but found nothing but a few school books and some debris. The orphanage had been blasted off the face of the earth. Eerily, a scattering of skulls had been washed out of a nearby cemetery. We also found a village cut off because of broken bridges and causeways. They had a terrible outbreak of dysentery and suspected cholera.

It will take years for these areas to return to anything like normal. For the men and women of HMS Chatham, Batticaloa has become a mission - but operational requirements will eventually force them to leave. Maybe other Royal Navy ships will take over, but that is for the politicians and sea lords to work out.