I think my nearest Nikon repairer is at Stoke on Trent and I'm in Stourbridge, so it's a bit of a jaunt. Fortunately, I've just managed to gently prize it free by using a tiny screwdriver. I'd lost my menu and all the other buttons had stopped working before, but now I've released it everything seems to be back and working. I think my best bet at the moment is not to use this button, but if something similar happens again then I'll have to get up to Stoke. I take it that it would need to be a Nikon dealer and not just a general camera shop I need to go to.
Thanks for your help.
Catherine