Since I sent the original email the problem has not recurred. I've
restarted emacs/gnus many times since then, and also rebooted, and
everything is now working okay. Thanks folks, I've made a note of
what to do if the problem recurs.

Thanks
Sharon. 
-- 
A taste of linux = http://www.sharons.org.uk
my git repo = https://bitbucket.org/boudiccas/dots
TGmeds = http://www.tgmeds.org.uk
Debian testing, fluxbox 1.3.6, emacs 24.4.1.0