Can anyone give some good advice re this problem ?

We have a combi boiler installed in 1992 - Vallliant VCW GB Glowworm Spacesaver. It still works very well and is regularly serviced by British Gas. But the water needs to be topped up quite frequently - at least once a week. We top it up to 1.5 when cold and it goes down to below 1.0 when cold at the end of the week. There does not seem to be any leaks in the pipes and radiators and it is much worse in the winter when the boiler is in use most of the time. Any ideas ?