Based on a true story, this film (along with Pat Jackson's Western Approaches) is probably the best film about the war at sea made during the war itself. Starting in a low-key, almost banal, manner, it builds into an inspiring drama of survival. Despite some limitations of production and special effects, it still exerts a grip seventy years after it was made.
The film opens in England, in November 1940. The oil tanker San Demetrio (registered in London, hence the title) is about to sail for America on convoy duty. In Galveston, Texas, the ship takes on a cargo of aviation fuel and picks up an extra hand, Preston (Robert Beatty), who claims to be a Canadian wanting to join the RAF. Spending his advance wages getting drunk in a bar, Preston belligerently (and unwisely) tells a stranger he is sailing for England on an old tub called San Demetrio. The stranger is, of course, San Demetrio's Bosun (Frederick Piper), and he immediately puts Preston (universally called 'Yank', despite insisting he's Canadian) to work painting the ship's funnel.
In mid-Atlantic San Demetrio's convoy is attacked by a German warship. The convoy's poorly-armed escort, the Jervis Bay, deliberately sails into enemy fire to allow the convoy to scatter before being sunk (Jervis Bay's captain was awarded a posthumous Victoria Cross for this action).
San Demetrio seems like she might be clear of danger when a shell punctures one of her oil tanks, causing a fire. The Captain gives the order to abandon ship and his boat is soon picked up. Another lifeboat, with the Chief Engineer (Walter Fitzgerald) and 2nd Officer Hawkins (Ralph Michael) on board, remains adrift. After two days the men are cold, wet, hungry and exhausted. Eventually, they spot a ship. As they approach, they realise it is on fire - it is the San Demetrio, somehow still afloat. After some discussion, the men agree to reboard rather than suffer in the boat. Men start tackling the fires and the Chief checks the engine room, eventually managing to power the firehoses. Spirits begin to lift, especially after the Chief takes a chance on lighting the stove and boils tea.
After clearing out the engine room, the Chief decides that despite the damage it might even be possible to get the ship under way. Hawkins consults the others and they agree to head East, hoping to reach Britain. San Demetrio resumes her voyage.
In the engine room, Boyle (Mervyn Johns), injured in the lifeboat, collapses and is put to bed. The Chief nominates Jamieson (Gordon Jackson) to take his place in the engine room.
To improve the ship's handling in rough seas, the Chief suggests transferring some of the oil to another tank to even the load, braving poisonous oil fumes to release the valves.
The men are cold and worn out, living on cold vegetables, but with fumes everywhere they dare not risk a fire. Jamieson scalds his hand in the boiler room, inspiring the Chief to use the heat to boil water for cooking. The men's spirits improve with the hot meal. Boyle dies and is buried at sea.
Finally, they reach Ireland's West coast, and the Admiralty sends a tug to assist them. Unknown to the crew, they stand to win substantial salvage money - provided they bring the ship home unaided. The Chief cheerfully gives the tug a two-finger salute, saying he can make more speed than the tug.
In a brief epilogue, a court awards salvage money totalling £14,700 (£3.75 million in 2015 values) to be shared among them; the judge praises their bravery. The crew requests that San Demetrio's Merchant Navy Red Ensign be presented to 'Yank' as a token of gratitude.
In a 1943 lecture, Ealing studio head Michael Balcon said San Demetrio London, would be "the best example of the final departure from tinsel which the film industry can make" and made the best use of the British documentary film tradition to tell a gripping story of courage, stoicism and resilience where the ship (with its crew of English, Scots and Welsh) represents Britain itself - besieged, damaged, but refusing to accept defeat.
The film also emphasises the unity required to achieve a common goal. Under Hawkins leadership officers do their share of the hard work, and key decisions - whether to reboard the stricken San Demetrio, whether to head for home or back to safer US waters - are put to the vote. Here is an obvious message about the ideals Britain was fighting for, and the kind of leadership the British expected when victory had been won: consensual and democratic, leaving behind the class structure of the past.
San Demetrio London is a benchmark for wartime realistic film-making and, as a true story, still packs a punch. It points the way towards post-war documentary-style films, such as Scott of the Antarctic, The Cruel Sea, and A Night to Remember, both in its treatment and the use of character actors rather than stars in the cast. Sadly, San Demetrio herself did not survive the war, being torpedoed off the US coast in 1942.