pressure falling?

Can anyone offer any advise. Our Worcester 24CDi RSF combi boiler is 10 years old and for the past 5 years we have had to continually top up the pressure by adding more water. We have our corgi reg man coming round but he just does not seem to know what the problem is. We top the water up during the winter months about once a week. I know this is not right as most of our neighbours have the same boiler but do not share this problem. Has anyone else had this problem and if so, can you help to resolve this for me.