Thanks dom.....I get the same results from the cap supplied with new radiator as I do with cap from old radiator

I saw the post re combustion gas tester...might try the litmus test first

What is doing my head in is the fact that I released the pressure when it was cold and then it developed more pressure over 4-5 hours while just sitting in shed!!!

Checked overflow level this evening...it is still sitting at 12mm higher so doesn't seem to be sucking it back as it cools....BUT.....radiator is still full ????