Potterton Suprima Boiler?

I have previosly stated that our problem is with a Potterton Primera when I should have said Potterton Suprima. Despite a new P.C.B. panel and pump the water boils up the expansion to the outside. The boiler runs perfect for maybe three to four days and then boills. I am a retired electrical Engineer wth a vast experience but not Corgi registered so even though I beleve the new P.C.B. is faulty I am on my own. Potterton are not prepared to talk to me despite the fact that I have more electrical knowledge that registered Corgi personel. Anybody any suggestions.