Vokera Compact Combi Boiler?

I keep having to top up the water in my system as the pressure drops cutting off the boiler. I have found that water is coming out of pipe outside which leaves the boiler by placing a clear plastic bag over it to collect the water (thus establishing I did not have a leak in my pipes under the floor).
Is there a quick fix, part i need to buy or an adjustment I need to make. I fixed my boiler the last time when the secondary thermistor went faulty leaving me with no hot water.

Alan Simpson
October 2008