Ok so not exactly the same as Sid's issue but realistically the most likely cause is obstruction or resistance from either broken or swollen runners, will check tonight and report back! Do you have a plan to fix it realbeale!?

Not at the moment. I can't justify the cost and time involved on something I rarely use. Unless it gives up the ghost entirely - but even then I'll probably just seal it up to stop it leaking. That's my plan at the moment. However, that's not to say I won't change my mind in the future.

RealBeale wrote:

Well, I found something interesting . The runner attached to the glass panel is broken in several places. There was even some loose plastic at the front of the channel in the pic. This is on both sides of the vehicle, the n/s has a piece of plastic broken off and floating around too.
If you apply slight upward force to the lower broken part of the channel in the picture, it moves really easily, suggesting it's not far from breaking off completely.

Click image to enlarge

Ok so had a look in the runners and can't see anything nasty in there! But there is plenty of new/fresh lube, presume spray grease, which means either the dealership was very thorough with their presale prep ( ) or they new the problem existed and hoped grease would sort it!