Thanks all for the offers of help.
The problem was the far track. The blocks in that channel would slow down, fall behind the front track and finally jam the window. We put an adjustable spanner on it to twist it and it would pick up momentum and go to the top.
Seeing this we put a permanent twist in and it would then almost reach the top. We put a washer on the top mount of the back track and we got full movement and secured it in the correct position.
Somewhere in my cars life, that track has got twisted. The metal plate across the opening is non standard and it makes you wonder why it was fitted or why it was felt necessary. It was never the greatest and it has seized up before when I was on the NC500, a shed load of silicon grease squirted down the channels got it going again. The plate coming away from the glass allowed a slight pivot of movement which probably helped it to work with damaged track as did the play in the central slider. As soon as I fixed these issues it made it totally unserviceable.
Anyway, it's now working. All this strain may have taken its toll on the motor but for now it's working so I won't go looking to change it at this time. As part of our desperate attempts to identify the problem we also removed the rollers and I honestly believe that they don't make any difference whatsoever.
Worst job we've had on the car I think.
Thanks all

did I do there situation