I ran the motor today on a separate 42v power supply and of course it would only run at about 500 revs but at that I could take a millimeter off diameter in a single cut without stalling. So the motor seems to have the guts for the job.. Speed drops a tad and current goes up but the torque is there despite low voltage.
I reckon the drive has fallen over . Now waiting to hear what suppliers will do about it .
Drive is supposed to be self protecting so should look after itself if a brush wears out (you would think anyway)