Got it sorted. After taking it to 2 different radiator places and having them both scratch their heads not knowing what was going on with the radiator/cap issue, I was flicking through the workshop manual looking in the cooling section for any clues and one phrase kept being used repeatedly which was "Be sure not to mix up the radiator cap with the filler cap". So I checked both caps and sure enough the seal part on the round one was about 10mm longer than the oval shaped one and after swapping them over it hasn't gone over 90 degrees since.
So as lame as it makes me feel I'll still own up and admit that it was me that stuffed up with this.
