Fixed the electric window
... and it wasn't a knackered motor.
The window was stuck fast. I took out the whole mechanism and it still didn't work. Only when I took the motor off did it spin freely on its own.
It is possible to get the black gearbox off the mechanism if you line up the holes in the toothed arm over the screws. That seemed to spin ok when attached to the motor. I took the opportunity to grease up the rest of the mechanism as best I could.
Once I had the window back in the door it ran up and down fine until I attached the guides. Once more it was stuck fast. On closer inspection it seems the white plastic clips in the guide blocks had started to pick up the surface of the black plastic guides. A good clean of all the plastic and a generous helping of silicone spray later and its working much better now.
Its still not perfect as it still sticks a bit on the way up. I suppose the scoring on the guides is fairly severe.
So to anyone else with window trouble. Don't dispare. It may just be stuck!