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Looks like 04OFF was right on the money with his suggestion about the problem being caused by the bolts which hold the winding mechanism to the inner door skin.
One bolt had fallen out and the other 3 were loose. You can see in the pic where the remaining bolts had been sliding up and down on the inner skin of the door, which was obviously causing the window to skip on the way down.
I also found where the guides on the bottom of the glass had been hitting the inner door skin also, so bent the skin away from this area to avoid any more contact.
Removing the door trim was a relatively easy job after I had a look at the "INTERIOR TRIM" section of an electronic workshop manual for the vehicle.
Hope the pics below of some of this job help someone else out.