Crowhurst was a weekend sailor who did what many have done. He set up in business in his sport. He designed and built a radio direction finder – the Navicator – and marketed it through his company, Electron Utilisation Ltd.

When the company began to fail, Crowhurst seized on the idea of competing in The Sunday Times Golden Globe as a way of getting publicity for himself and his Navicator.

The film – which is superbly researched and presented as a documentary – uses Crowhurst’s original 16mm films and tape recordings to reconstruct his bizarre journey. Much of this astonishing archive has not been seen for over 30 years and was only recently rediscovered by the production team.

Deep Water is an apt tile, as what must have seemed A Good Idea At The Time turned into a nightmare as Crowhurst faced a trail of difficulties, culminating in the realisation his boat was unseaworthy (he stopped in Argentina to patch it up) and unlikely to get him through the Southern Ocean.

His ruse of dallying in the South Atlantic to fall in behind the other boats as they came back up to the finish was dashed by the number of retirements.

From eight starters, John Ridgeway retired in Recife; Chay Blyth retired after a severe storm off South Africa; Bill King retired his Chinese junk, Galway Blazer, to South Africa. Loic Fougeron retired after a storm on October 31 and Alex Carozzo was forced to retire in Lisbon where he had emergency surgery on a stomach ulcer.

That left the leader, Bernard Moitessier, Robin Knox-Johnston and Nigel Tetley sailing up the South Atlantic in that order. A fourth place would be ideal for Crowhurst – his logs probably wouldn’t be examined and he’d get away with it.

Moitessier was the first to go; the Frenchman decided he preferred the sea to the land, so he turned south to go round the globe again(!). Then Tetley’s boat sank under him.

And Crowhurst – now in second place - had nowhere to go.

To finish second to RKJ would mean he would win the £5,000 prize for the fastest circumnavigation. And his logs would be examined.

He would face total humiliation, bankruptcy and the loss of everything he had; probably including his family. So he laid out logbooks and sound recordings in his cabin before stepping off the side of his boat after 243 days at sea.

Crowhurst’s body was never recovered. Teignmouth Electron lies rotting on Cayman Brac in the West Indies.

In truth, he never stood a chance, right from the beginning. He should never have set out in the first place (wearing a jacket and tie!). By the time he started the race, he was already trapped.

Deep Water sets it all out, through interviews with his family and the people who helped him and, unwittingly, trapped him into taking that final step into the ocean.

He had nowhere else to go…