Sleeping afloat on the Thames in the shadow of Tower Bridge seems like a good idea for a school visit to central London. But the chance for pupils to live as sailors once did on an historic battle-cruiser which saw intense action in the Second World War is even better - experiencing first-hand what it was like to eat, work and survive on this huge, cramped, and now highly evocative, relic of British naval tradition.

The Year 8s from Cirencester Deer Park school (an 11-16 comprehensive) paid pound;130 for the privilege of staying aboard HMS Belfast for four nights.

Four staff and 35 pupils are on the visit and on their last day they reel off the places they have been to: the Natural History and Science museums, London Eye, Covent Garden, Tower Bridge, Imax cinema and the dance spectacular Stomp, to name a few attractions.

But today is the day for the ship. They’ve slept in bunks in comfortable dormitories (once the officers’ accommodation) and eaten a ship’s breakfast. They are seated in the education suite, once a hangar which housed Walrus seaplanes. They are listening to Ngaire Bushell, learning and access officer, telling them about HMS Belfast and her history. A lot of showing goes with the telling and there is constant reference to a picture of the ship in her prime.

Built in the city she is named after and launched in 1938, HMS Belfast was a warship with a punch. Ngaire explains the weaponry first: huge shells which reached targets 12 miles away and could be fired at a rate of eight per minute by the six-inch guns.

The gunners wore white anti-flash hoods and long white opera-style gloves.

Slightly anxiously the students try them on. “These are not the authentic 1940s article,” Ngaire explains, “those had asbestos in them.”

The guns were no defence against a magnetic mine which blew up HMS Belfast three months after the war started. She came back into service in 1943, newly-equipped with radar, and escorted the Arctic convoys to Murmansk, delivering vital equipment to the Soviet army. A film of the appalling seas, which froze as they washed over the decks, deeply impresses the pupils. Sailors wearing duffel coats had to remove ice from the superstructure in case the ship became top heavy. The coats are tried on (and pronounced “hairy”). They are warm on one of the hottest days of the year, but it is clear how inadequate they would have been when saturated with water in temperatures far below zero.

Ratings’ and officers’ uniforms are tried on, and one boy swings himself with Tarzan-like ease into a dangling, stringy hammock. After that we head round the ship in small groups. On active service in wartime, the 950 crew ate, slept and relaxed as best they could in almost prison-like proximity with each other. Ratings slept in hammocks, far preferable in rough seas.

Everyone worked four-hour shifts. Christmas dinner at sea meant, if they were lucky, beef sandwiches. The sheer difficulty of getting on with hundreds of other men in uncomfortable and often frightening conditions, and with hardly a moment’s privacy, comes as a revelation to these 12 and 13-year-olds.

On the bridge they sit in the Admiral’s chair, from which he surveyed the entire fleet, and then they go down to the lower bridge, where the captain steered his ship and directed his gunners. The most interesting and claustrophobic place on board is the magazine room, deep in the bowels.

Hundred of shells are stored in drawers and hang on carousels. Each has to be armed with cordite before it is sent up on a hoist to the gunners several decks above. If the ship is hit, and the magazine threatened, the room has to be flooded instantly and all nine gunners would perish as the water flowed in.

On deck, flags and semaphore are demonstrated. The sun shines warmly. We are at the historic core of London. The pupils use words such as “excellent”, “brilliant” and “phenomenal” to describe the trip. From their ship day, they remember, above all, “what it meant to be a sailor” - the cold, the claustrophobia and the pride in their ship.

At the end of the day one pupil demonstrates the remarkable loyalty of sailors to HMS Belfast. Emma Hope’s great uncle served on the vessel and became so attached to her that, one day, his ashes will be scattered in the Thames from her deck.

www.iwm.org.ukhmsbelfast; to book, contact the education office, tel: 020 7940 633663486323; email: hms-edu@iwm.org.uk