Thanks very much for all these suggestions; I think that by a combination I shall get it to work. There isn't a lot of room for moving back and forth because of the downwards slope of the roof at the back, and the fact that I fit a rack as well which needs the bars to be a fixed width apart.

Ron: I think the door is a little off kilter; it doesn't shut every time when you slam (I mean push and let go) but usually does if you push-all-the-way shut.

Tom