I've bitten the bullet and had the motor replaced. It was rated at 250
watts, 17rpm and 40Nm torque (figures which seem to imply an efficiency of
about 35%). It was about 60 cm long and only about 5 cm in diameter with no
sign of any gearbox.

It goes against the grain to admit it but I have so far been unable to get
it apart to examine it in more detail.

Stephen