Talked to a Bletchley park guide last Saturday, and he gave a bit more info about the U-boat. Seems the two guys aboard were about to leave the sinking boat when they noticed a 4 rotor Enigma bolted to a desk in the captain's cabin. They were determined to take this as well, so hunted for a spanner and were unbolting the the thing when the boat went down.
However the documents they had previously passed up to the guy who stayed on the conning tower, were immeasurably helpful in braking the latest code.