Sounds like a pressure test may be useful, whereabouts are you??
Just noticed it says Ireland on your thingy. You should be able to get a local garage to do it and you will find out very quickly if you have a leak or if it's a head gasket issue....changing stuff in the hope you will sort it is a bit of a hit & miss affair & can cost you more money than getting a proper diagnostic & fixing the issue, my old college tutor had a saying "do it once, do it right"