Well today a clear bag labelled 50gms with a funny white powder arrived in the post.
Luckily it also said "PTFE" and not "Charley".
I applied the powder to the channels with a paint brush, no difference.
I looked at the channel and frame at the bottom, and it looked to be binding on one side of the glass.
Being scientifically minded I reckoned it would be easier to bend the aluminium frame rather than bend the glass made of unobtainium.
I took the window frame out of the door took one of the felt channels out and the frame was definitely more curved than the glass.
I took the glass out of the frame, put it in a very safe place so as not to drop it on the garage floor, and carefully made each side of the aluminium frame less curved.
It's a miracle, the glass now slides up and down the frame easily.
It took me less than 2 hours start to finish to take out the frame straighten it and refit it. I should have checked this before I re-assembled the door. Hindsight is a wonderful thing.
Now I await the arrival of a replacement motor for the other side, bought on eBay. The postage from the USA was greater than the cost of the motor !
It seems I had the same problem as Kendo above.