The vibration was no-where near as bad as with the tailshaft - but quite definitely present and at the same speeds that I am getting the vibration with tailshaft attached.
So the motor is apparently the root cause.
I spoke to Jeff at Precision Balancing (who made the driveshaft) this morning and he asked where the motor was made then indicated that he does lots of Induction Motors for higher speed and the nominal rotor balance is only ever "just enough" for their nominal running speed - in my case 1500 RPM. There was some complication to do with a language barrier around the time I was discussing rotor balance with Qin Wei (the motor manufacturer in Taiwan) - only that they indicated that it would be fine to 4000 RPM..
So I'll strip the engine bay, drop the motor cradle, remove the motor then remove the rotor and take it up to Jeff. He also warned me to check the bearings for a speed rating. They are NSK bearings but I'll have them out anyway when the rotor comes out so I'll check.
Moral - balance those rotors.
A point here is that this probably wouldn't be a problem in a FWD where the motor was hard coupled to the gearbox. The rubber mounts in my case allow some movement that appears to be amplified by the tailshaft.
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