Everything back together now, watched the pump to make sure it was turning ok, seems fine, that gasket looks like a good idea. Can't test it yet cos it's stuck in the conservatory, and if I start it and the house fills with smoke (or even hopefully just exhaust fumes) there could be trouble
. I'm not overconfident I've fixed anything, but we'll see. It's easy to see how it can look like the pump is in there tight against the case, because the worm makes it feel like that even when it's still a mill or so from home, so maybe