Doh Forgot about the rad, could be a pin hole in the rad.
Not saying Dogma is scaremongering, I heard all the stories of the head gasket going or warped heads, I also spent ages refilling the system trying to cure an airlock, but as I said it was a tiny pinhole leak in the end.
Just get the car up to temp and pressure and spend time looking for leaks. I have a head gasket sniffer test I bought and never used, happy to sell it on to someone who needs it.