Had a bit more of a play. Disconneced the cartridge - hum goes. So it's
apparently being induced into the cartridge itself, rather than wiring. No
large transformers anywhere near - and it's more or less the same on the
workbench, in a different room. Next thing to try is removing the mains to
the turntable, I suppose.

--
*A chicken crossing the road is poultry in motion.*

Dave Plowman London SW
To e-mail, change noise into sound.