Boiler expansion pipe blowing back?

Hi! I have a two-storey house, the boiler was originally on the ground floor. Seven years ago the boiler was replaced and relocated in the roof space; the system was changed from gravity-fed to a mains pressure sealed system. In the last six months the pressure has started to drop and now drops fromtwo bar to one bar in three days, the heating compaby say that there is an underfloor leak (solid floors) and the answer is to dig up the whole of the ground floor to locate the problem! So I have changed the system back to gravity fed on the assumption that the pressure and therefore the leak will be minimal, I will add a leak seal to the water. However, the vent pipe overflows into the header tank with much hissing and steam when the boiler shuts down. The header tank is 5ft above the boiler and the vent pipe is 22mm. Can anyone suggest what I have done wrong please? Many thanks, David.