halstead ace high over pressurising?

I have a 5 year old Halstead Ace High boiler recently when the central heating is running the pressure has been rising from 1 bar to 3 bar within the first hour of the central heating coming on. The pressure is releasing through the prv like it should do, i have no visible leaks and have checked the expansion vessell which is charged to 1 bar. it seems that the boiler is only happy running from a static 0.25 bar which isn't right, a mate of mine seems to think i need a new expansion vessell but when the pressure is released from the expansion vessell through the charging valve there is no evidence of water contamination.has anybody got any ideas, thanks