This is not what you can call "tactical", but here goes.... Sink the biggest ships, and ignore the rest

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But in all seriousness, I would tail the convoy at a distance and send reports to BDu, and wait for night to attack as its only 1939, you shouldn't worry about the escorts as they won't have ASDIC. Just pick you targets carefully and don't bother with the deck gun - that's asking for trouble, save that for lone merchants.
I would go for the tankers, as they are the most valuable, and use 2 torpeodoes on each to make sure each is sunk. Once you've launched a salvo, dive imediatley and go silent, this should shake off any of the escorts easily, and re-load. Then get yourself in postion for another attack, and repeat unitl you've had enough or out of torpedoes - Enjoy
As for reporting to BDu, when you send a contact report, you report the whole convoy and its approximate speed and course.