the guy who had the car before me has recently flushed the rad and just put water in it, i knew that it needed coolant putting in.. i guess there could be an airlock but i dont think that would cause the gauge to show the car overheating would it?

I guess the best thing to do is as rich said, drop the water from teh rad replace that with coolant and run the car with the cap off to remove any air trapped in the system?

Then replace the stat aswell and see if this cures my problem?