I think I have cracked it. - Alignment.

There was a wobble in the wheels, where the sprocket was bolted to a spacer onto the hub. Also the drive sprocket and the wheel sprocket were not aligned precisely. Now cumulatively this can't have been that much a few mm. But I ran it full speed forward and back a few cycles no problem. Going from max power to stop does generate a clunk but that can be the chain going from teapnsionnone direction to the other.

Again I'll add photos soon at the momentum need to get back and repeat process on the other side!