Oh dear! I have now managed to sort it out, I was doing something stupid after all. You were right Gnobelix, I should have double-checked the brush blend mode. It was on Normal but I have just changed it to Overlay and painting worked. Strangely enough, now when I change it back to Normal again painting still works, although it didn't work until I actually changed it for the first time. Very odd but I don't care now.
Thanks very much for your help.
Cheers
Faraway got a reaction from Gnobelix in Masking not working