o I'm going to follow Brit01's example and strip the motor down!
It's really simple.
Take out the whole mechanism (that's the fiddly part).
Undo the nut that holds the metal lever (that rotates) directly on the motor shaft. There is a fiddly clip to remove on this shaft also.
Remove the plastic cover, there are some clips that come off easily.
Underneath you'll see the worm gear and large gear. You'll find a lot of old congealed grease.
Then there are 2 screws that hold the motor casing to it's mounting.
Unscrew these and slide off the motor casing. Then you can pull out the internal part of the motor from the magnetic casing.
You'll see 3 carbon strips that make contact with the copper cylindrical rotor. If this is grey/black, finely sand it until it's a shiny copper colour.
Inspect the carbon strips making sure they have enough pressure/length to make contact with the motor shaft.
Re-grease the gears, re-assemble and it should all work nicely again.
Let me know how it goes.