Finally managed to have a go at the roof this afternoon with the help of my long-suffering neighbour. We did as Flybobbie suggested and eased the roof panels up, propping it up with a length of timber placed between the boot floor and the underside of the cockpit roof. With the roof roughly level, I tentatively pressed the roof close button and the hydraulics whirred into action. Halted the operation after a moment so that we could remove the tyre inflator which had caused the jam, and then checked that everything on the roof gubbins was in roughly the right position, which it was.
So we carried on closing the roof and to my surprise it closed perfectly. Tried it again all the way down and back up again and it works fine! Have to say that I'm completely astonished by this - I was expecting to have to trailer the car away to somewhere very expensive to get it fixed, but it seems to be absolutely perfect - we can't see anything out of line or not as it should be. As a precaution, for the next week I'll only be dropping the roof when the car's at home or somewhere else convenient to leave it should the operation fail, but at the moment the omens look good.
Thanks to all for the suggestions and sympathy - it was very helpful. I'm now going to the pub and I may be gone some time!
Freddie