2000 Fiat Punto Replace head Gasket

I broke down a couple of weeks ago and had to call the RAC out. the RAC guy had a look at the engine and told me he suspected the head gasket had blown. I had the car repaired by a mechanic who charged me £300 to replace the head gasket parts.

Thinking I was safe I headed off to Yorkshire at the weekend - only to breakdown again (on the motorway) again I called the RAC who said again he suspected it was the head gasket. On monday I took the car back to the mechanc who intitially fixed it for me - . Tuesday I had a call telling me it is the core plug which needs replacing and although he has ordered the part it will take a few days to arrive. Obviously I am going to be charged again for this repair which I am totally unhappy about. My argument is; I feel he should have spotted any other problems whilst mending the head gasket? I feel that this mechanic is taken advantage of me as a woman as I do not know the first thing about the mechanics of a car. I know I don't have a temperature gauge but the temperature light came on the second time I broke down - the first time I had no inclining until the engine started making funny noises. Am I right to be suspicious of this mechanic or am I just unlucky? thank you for your time Marie