Thanks for all the input-this really is a helpful community.
I can see where I'm going wrong-I need to let the convoy come to me by positioning myself correctly instead of trying to chase it. I'm depleting my batteries by running too fast and also making a lot of noise in the process. As for radar, well I missed that trick: I thought it came in later. Still, my lack of awareness was good for my immersion I suppose, that is, radar coming as a nasty surprise. There weren't many uboat skippers saying "It's 1st March 1941, the Brits are due to start using radar today"

.
I'm still left with a sense of uneasiness about how easily I sank three escorts . Perhaps I just got lucky. I would have thought realistic tactics would involve zig-zagging as they approached the most dangerous end of a uboat and also working as a pair to divide my attention. Oh well, if aggressive tactics work I will keep it up-better than sitting quietly in the deep waiting to be killed by dcs and there is no doubt about the deadliness of the GWX escorts once they've got you "on the ropes". It must be playing all those flightsims that makes me addicted to a quick shoot-out.
Seriously though, thanks everyone for the advice.