Voicecoil,
I thought I'd been quite careful with the wiring: the BOB is in it's own aluminium box, the outputs to the drivers are 4 core screened, the cables from the drivers to the motors are screened, the PSU for the BOB (it's the original ATX supply in the PC case that is my control box) is a separate unit from the motor PSUs, the VFD is separately mounted on the wall, the spindle power cable is proper screened cable designed for the job.

The weak point is the sensor wiring which was not all screened cable for the old microswitches and has not been totally replaced yet, but will be soon. All the grounding of every component in the control box will need to be looked at again as well. No going down to the pub this evening for me!