Don't worry Easty, that happens quite often. I recently sunk two freighters in Freetown harbour by torpedo attack, they really slowly sunk, but I wasn't credited.
Is has something to do with saving, you should never save a game in combat. If you see an enemy, save the game, attack it, wait until it sinks, get the credit, save again. Not in between.
This error is driving me mad, too!
