Just dismantled the other two drills and they had the same issue. If you took them apart during the grub-screwing process Tony, then chances are you just put the gears back in the wrong place, but if you didn't open up the gearboxes at all since getting them, then I'm kinda shocked at the lack of quality control and that these are actually making it to customers.

This is what the metal gear stage looked like in all four motors:



Those three gears should be fitted onto the metal pins so that they can rotate but keep their place in the overall layout. But here, they are free to move in or out from the centre and do not engage properly when turning. Pretty useless as a cordless drill let alone a robot motor.