potterton boiler trips main rcd in house, what is wrong?

I have a potterton Boiler Suprima 50 (which has had the pcb changed about 12 months ago) How ever a new problem is no occuring. The boiler fires up, gas burns and then runs for 30 - 60 seconds at which point the main rcd in house trips. What is the problem and is it a known fault? My plumber is trying to sort it but seems at a loss......meanwhile my wife and kids (and me) are freezing.