Rob
I have no answer, but can at least say that you aren't alone, at least if you have an O2 SIM.
Take a look at:
https://discussions.apple.com/thread/4032787?start=0&tstart=0
and join the huddled masses struggling with the same problem. O2 is the problem, despite my 30 minute conversation where they ended by saying "Nothing to do with us, guv..."
Regards
Nick