After disassembling it, it was apparent the motor + bearings were fine, just loose crank bolt on the square taper axle had completely nuked the crank arm. I only had a small torque wrench (up to 24nm) so guessed, and seemingly I didn't guess very well.
That said.. one new crank arm later, and a torque wrench with the appropriate range, did it up to 40nm and the cranks are so tight on they won't even turn! hmmmm. Can't see that I could have cocked up anything in the reassembly on - black spacer plate, castle nut using the supplied tool, then crank arm...
Any thoughts? I'm going to back it undo it and try it again at whatever turns out to be the highest torque setting I can manage without it binding.