[peterg]

I have now identified the problem but I have no idea how it occurred or how to fix it.

The window has come out of the sliders and is floating on the inside of the door, still attached to the winding mechanism.

When I last used it the operation was notchy but the glass slid evenly up and down (still does), so how could it have come out of the slider? There doesn't appear to be enough space for it to fall out and certainly doesn't appear to have enough clearance to get it back in.

How did it come out and, but more importantly, how can I get it back in?