In October 1939, just one month after the declaration of war, Prime Minister Winston Churchill was distraught when he learned that a German U-boat had penetrated the Fleet's safe anchor at Scapa Flow in the Orkneys and had sunk HMS Royal Oak with the loss of 833 men, a large proportion of whom were boy sailors.

For years controversy has surrounded the sinking. While many publicly accepted that the ship had been sunk by a U-boat, the Royal Navy continued to insist that it was the result of an accidental explosion in an ammunition store.

Now author David Turner, whose uncle perished in the tragedy, has researched the evidence from a modern perspective.

Soon to be the subject of a film for television, Turner's fascinating book re-examines all the major aspects of this terrible catastrophe and interviews some of the few remaining survivors.

We learn about the German spy allegedly living in Kirkwall, Orkney, who passed on crucial information about the lack of defences and Churchill's stiff rebuke to the Navy Chiefs of Staff about the lack of anti-submarine defences at Scapa Flow.

He also examines the claim that on the night of the tragedy, several survivors, covered in oil and badly burned, swam ashore but locals were prevented by police and coastguards from descending the cliffs to help them.

Of the 833 sailors who died, many were aged between 15 and 17 - a fact that was kept secret at the time and which led to new Admiralty regulations that boy sailors were not to be present on ships on active service.

We also learn that submarine commander Captain Gunther Prien became one of Germany's first real heroes of the Second World War and was given lunch by Hitler on his triumphant returm to Germany.

Ultimately, the Royal Navy had been caught napping with their defences down by a submarine crew only too happy to exploit their mistakes.

This is a tale of cunning and daring by the Germans and one of unimaginable anguish for wartime Britain. When Churchill was told of the sinking, he broke down and wept. 'Poor fellows, poor fellows,' he sobbed, 'trapped in those black depths.'

(Argyll, paperback, £7.99)