Water pressure drops, water expelled through overflow?

My boiler is a Glow worm Swift flow 80e Fanned flue Combination. 10 years old but not really had much use.
Each time I use the central heating, with the water level reading set at 1.5, water is ejected through the outside overflow.I know that is definitely where it is going as I set up a bottle to collect it. Once the boiler has cooled I can see that the pressure has dropped to 0.7 or so. It means I am constantly refilling to 1.5. Several plumbers have come and changed small bits, at vaste expense, in an attempt to rectify this problem but to date no luck.Any advice gratefully received so that I can at least sound as if I know what I'm talking about to the next plumber! Should I give up, do I need a new boiler? If so what would you recommend fo a one person household.Thank you.