Thanks, gonna give this a try tomorrow. I've not even had the car a week and after driving it around a bit more today it seems that starting it cold it revs at about 1000rpm then after it's warmed up and I stop at lights etc it revs high at about 1500 and starts to fluctuate a lot. Maybe that's why I didn't notice it on my short test drive.
If it doesn't help I will have some words with the Dodgy 2nd hand car dealership and see if I can get them to sort it