Loss of pressure on combi boiler?

have a vokera unica combi boiler if I fill as per instuctions up to 1.5 bar the pressure drops to 0.8 bar very quickly (approx 10 mins) then just stops at 0.8 bar, the heating and water all work fine but cannot hold pressure any higher than 0.8 bar. If was a leak ume would drop to 0 and not just hold at 0.8bar??