I am working fro feeble memory here as it is a long time ago that I read the book, but did HMS Graph (ex U570) not sink a uboat?

There is another RN submarine while submerged which sank a uboat in the Bay of Biscay . The Cpt was in the head when the uboat was seen and rushed out to the control room, the lead time from spotting to attack range being extremely short. The RN Cpt quickly setup the attack and hit the uboat. He then looked down and noticed in his haste that he had not fastened his trouser belt, thus making him the only submarine cmdr to sink a vessel with his trousers around his ankles.