that magic plug showed me the live and neutral were reversed on the set of sockets around the fishtank, so i looked inside them all and all were wired properly. still i suspected that socket that has been showered in salty fishwater once or twice had a short inside it, and after another trip to the electricans, replaced that and another socket. guess what? yes that's right, the fault was still there.
so i followed along the chain of fault-showing sockets until i found one that was also attached to a vertical spur of some kind. disconnected that, all power gone, so that spur was the source for my dodgy sockets. all seemed to be wired properly, but i followed that vertical wire and discovered it was reversed at the other end. it musta been like that for at least ten years, i just never touched two objects before that were wired into two different spurs in the same room. lucky escape.
end result - confusion by a mix of modern UK wiring (live is brown, neutral is blue) being used alongside older UK stuff (live is red, neutral is black). sheesh. and don't forget that inside these ****ing machines they tend to have US wiring colours (different again, this time live is black) and that they may also have been imported from some other country where the colours may be different again.
i'm just happy to have sorted it (no more AC voltage between sockets or between machines), to have not killed myself, and not to have to cancel tomorrow's party.