Great work SubConcious!
I was on my way back to base with no torps left and a locker 3/4 full of HE rounds when I came across what looked like a completely unescorted convoy slowly steming for England at around 2330, (early 1940 about 200km southwest of the Irish coast).
Had a good look at them from a distance, (not bad visibility but no moon), then decided why not give some fo the bigger prizes a few rounds to see if I could sink a few.
So I moved in for the kill and was just lining up on a T3 tanker when from out of nowhere I get the "We;ve been spotted sir!" search lights stab out of the darkness and I hear the whizzing of about 5 shells in quck succession going over head and see a few splahes down range of me.
Sure enough there was a verdammt Auxilary Cruiser taking shots at me.

I swung around the bows of the T3 at flank trying to keep the tanker between me and the cruiser and kept up the firing to see if I could at least get a kill.
Anyway after about half and hour of mad manouvering and trying to keep up a steady fire on the T3 i ran out of ammo, the T3 was still floating but on fire and the crusier was closing fast! so i dived and sat a few km off to see if the T3 would go down whilst the cruiser took off with the rest of teh convoy. hung around for another 2 hours game time with no joy and decided i'd had enough when I got a hydrophone contact on a warship closing fast.
Assuming it was a destroyer i surfaced and took off at full speed to get some distance between me and the tanker. Very exciting stuff but ultimately quite diappointing because the T3 was still floating there as I disappeared over the horizon.