Happened again over weekend, and wouldn't sort itself out this time, so I took the opportunity to had a good look araound, I cleaned some various connections and then found that the throttle assembly was fairly dirty and wasn't moving freely enough, causing it to not always click the little button on idle.. I'm assuming this was the problem, so I cleaned it all up, and is now moving back into position correctly.
Seems to have sorted the problem, or at least it hasn't happened again!
Thanks for help
Wilson