Just to update this problem in case anyone searches the forum: - in my case the problem was solved by tightening the nuts that attach the cable to the window pane.
For some reason the cable was slipping through the clamping mechanism. There seems to be some natural travel of the cable from fully up to fully down. I pressed the down button to let the window come down (which due to gravity it was happy to do) and kept on the down button whilst the cable slipped - until the motor stopped winding. I then tightened the two nuts and - hey presto the window goes up and down like a charm. I'm just relieved it wasn't an electrical fault.
