Well, I decided to take out the Destroyer moored at the western end, but as I got in close and upped periscope I realised it was actually 2 subchasers moored there. Not worth wasting torps on - especially when I realised that there were 2 merchants and 2 tankers inside the harbour
So doubled back, waited till nightfall, to take out the patrolling DD and unbelievably, both torps missed!! 1 mile, slow speed, perpendicular course, and even with auto-targeting I missed with both

So had to wait, fired another 2, and of course the first one hit and caused a giant explosion which sank the destroyer straight away. So I'd wasted 4 torps on 1 DD, which I would soon regret....
Went into the harbour at periscope depth because of the 4 subchasers & gunboats moored there. Used 6 torps taking out 1 tanker and 1 freighter. Back out and looped round to the east, fired my 2 remaining torps at the tanker moored outside the eastern pier, which of course wasn't enough to sink it. So I surfaced to finish it off with my deck gun, but for some infuriating reason my gun crew insisted on firing on a freighter on the other side of the pier, all whilst the armed freighters and gunboats were taking pot shots at me.
**Eventually**

my gun crew sank their freighter and turned on the damaged tanker. Once that went down I was able to dive and get away. 2 tankers, 2 med freighters and a destroyer. Not a bad haul, even if I was blundering around in my n00bness.
So how do I get my gun to target who I want? I tried locking onto the tanker with my TDC, but they still kept firing at the freighter. I also tried manual control, but in the dark I found it hard to see if I was hitting the target or not. Really short shots I could see the splash, I couldn't tell when I was overshooting or hitting the target.