A most excellent test - simple enough and very revealing.
CDL motor is close to open circuit (1.5MΩ) so that is one duff motor.
The superlock motor is a bit below the lower end of the tolerance so is probably OK.
While the door card is off I checked what voltage is being sent to the CDL motor on central locking - a very short pulse that my meter showed was a volt or so, but it was over as soon as it began.
Good used latch from East Coast 4x4 is probably the way forward...any favourite used parts suppliers that I should be aware of?
MGF motors from the same year have the same motors at a fraction of the cost. The motors just clip in and out.