I managed to track the film down on DVD a few years ago. Never tire of it. Although loosely based on the HMS Truculent sinking, it is VERY close to the HMS Thetis disaster of June 1939 which occurred in Liverpool bay. Due to the a tiny blockage in the test tube which tells if a tube is full of water or dry, the number 5 tube door was actually open to the sea and the forward torpedo room was flooded. Only 4 men managed to escape from a compliment of 104 and 99 perished. The sub was salvaged and beached nearby where many bodies were removed, she was then refloated and dry docked in our home town of Holyhead, where the rest of the men were recovered, having been on board for some five months.
My father recalls the funerals of many as he lived opposite the cemetery where many lie today.
I often think of the final scene of the film being so very like what it would have been like on board, with hope fading as quickly as the air supply. The failure to raise the sub is accurately portrayed and has often led to speculation that in order to help the crew escape would have meant cutting into the hull and with war declared, the crew were less valuable than the submarine.