My Vokera 28 combi boiler keeps over pressurising?

I have a Vokera28 boiler fo about 2.5 years and just recently it has started to over pressurise causing the relief valve to constantly blow off down the side of the house. I have drained the excess pressure off several times but within an hour it is rising again, even without any heating on. I have recharged the expansion vessel with a bike pump to 0.8 bar as given in the fitting instructions, but it is still raising to max pressure within hours.
Any advice gladly accepted.