[Arran]

 Have you tried disconnecting one of the filter caps yet? Either the input one, before the field coil, or the output one after? The hum should get louder or change if either one is disconnected. I'm wondering if one of the filter caps may have a bad connection on the negative side, or that you may have confused common negative with the chassis, common negative is the center tap of the H.V winding on the power transformer. On part #6 the positive end of that capacitor must go to chassis, the chassis is more positive with respect to the center tap, parts #6A and #6B also have their negative end going to the center tap, but not the chassis.
Regards
Arran