I think, hope i might of got to the bottom of the boiling over mystery. I took off the water pump and found it to be ok last week, but whilst the readiator was out deciced to get it repaired as it seemed the pressure had made the header and lower tank break away from the core and weep. Once i showed it to the guy at the workshop he said it was goosed and totally blocked which he seemed to think that was 90% of the reason it was boiling over which the original blocked stat may have been coming from. So i have had it recored for £94 + VAT and i am going to refit the stat and give it a whirl on Monday if the weather forecast is right as i have to do this outside.....so here's hoping.