Fixed at last (until the next problem!)
At last it looks as though it's sorted. It turns out I have dodgy air mas meter (or Naff MAF as someone referred to it on here once). It's a bit strange that it didn't show up the last 3 times they did a diagnostic on it.
I have replaced the MAF and the car seems to be running perfectly again. Almost forgot what it was like to drive a car that wasn;t chasing revs and jerking every 5 minutes.
When the put it through the diagnostic it showed up a failt with the altimeter (I didn't even know that cars had altimeters). However, this doesn't seem to be affecting the driving so its not too bad.
I would recommend looking at the Maf before cleaning the throttle body (unless you're cleaning it yourself) as apparently you can test it by simply unplugging it to see if the problem persists.
Thanks for all the suggestions opinions.