Thanks for that Martin.
I've had the idle valve off to give it a clean out. It wasn't sticking and there wasn't much dirt in there either. There doesn't look to be a vacuum leak from what I can see, but the next step looks like checking it on the computer and seeing if it throws a fault code up. Hopefully it will, just so i can finally get this annoying problem sorted. It drives well enough, it's just idling with the odd auxiliary that the problem occurs.
Thanks for the reply though, I appreciate your advice
Simon