potterton netaheat 10-16?

Help!!!! Boiler comes on in normal way, but every now and then , it starts to rumble and bubble scalding hot water up into the expantion tank in the loft. Its melted the ball valve. Plumber replaced it and programer /room stat /tank stat & mid posion valve. pump works he even flushed the system through.
Fine for two days, but started doing it again. He has now run away from it. and i canot find anyone to even look at it. Please can anyone Help with some advice