Found it, my luck
The documentry is named: "Kriegsmarine 1914-1945" the fact mentioned in around 38 min. out of 1:21:19
German title: "Die Geschichte der Deutschen Kriegsmarine 1914-1945 "
One of the facts:
November 1939: One u-boat attacked HMS Nelson on board which was Prime Minister W.C.
U-boat fired 3 torpedoes to HMS Nelson, all three hit the target but none of them exploaded.
What consequences it could have to British Admiralty, leaves us guessing..