Thanks for those suggestions - on further investigation it seems that I have a 50 Hz hum on both left and right channels. I'll check the state of the connections on the cartridge tags. Presumably I could check with a multimeter for continuity between each of the cartridge tags and the RCA connectors signal pins and ground shields? Would this tell me if there was a break in the internal wiring of the arm?
The induced magnetic hum might be a consideration - the power amp is a Krell KSA 50 and I know that these have a fairly beefy power supply and so this might be the source of the problem. The LP12 is on a Quadraspire wall mounted shelf and at present the Krell is at roughly the same height as the LP12 on the top of an adjacent rack about 25cm to the right of the turntable. I normally would house power amps lower down in the rack, but the depth of the KSA50 is such that the speaker terminals on the rear panel foul the back plate of the rack, so I had to put it on the top shelf. Over the weekend I will try moving the Krell away from the turntable to see if it is an induced hum. If there's no change, I will be out of ideas!
I understand your reluctance to take the risk of receiving the Zeta by courier - I read some of your earlier experiences with companies like Hermes! I still have the hinged wooden box that the Zeta was originally supplied in - doesn't that provide sufficient protection for transit?
Thank you for taking the time to have this conversation with me.
All the best