Thank you for answering. The kiln is very basic, no kiln sitter, just the controller shown in the attached phot. I changed the fuse which has fixed it, but I don't know why the fuse would have blown - maybe it was just old. I waited until it had cooled and had a look inside - everything looks ok, just half-fired! I have now turned it on again and will just watch to see if it happens again.
Julie