Get a garage to do a flow test on the radiator.
I had a Mazda that was losing water. Paid to have the head gasket replaced. Still had the problem. A flow test showed that there was a partial blockage of the radiator. One new radiator and the problem was fixed. Pity I did not know about the flow test before I spent the cash on having the head gasket done!!!!!!
AcmeUK
February 2009