After a conversation with Chris (in which he showed me it worked fine for him) I checked other browsers and they were OK. I even checked Firefox (which was giving the problem) on another PC and that was OK too. I restarted Firefox, and it now works OK.
I've no idea what the problem was to start with, and I don't think Chris made any changes to my set up ... so it's complete mystery.
My apolgies for not posting to say the problem had gone.