If you have no signs of a leak and youre losing pressure, youre pressure relief valve is probaly letting by, this is a safety valve which allows water to escape outside safely if the central heating pressure becomes to high, it will normally be a 15mm copper pipe going through the wall outside it will just be kicked back on itself. if you know where this is or can trace the pipe to outside, if the wall is wet or has an orange like stain or you can see it dripping that is most probaly youre problem, all so when you refill the system you should vent the radiators just incase theres any air in the system.
hope this is of some help, sam(s.m.wynne plumbing and heating services)

Hello Derwyn,

Your pressure issue is either a leak on the existing heating pipework, or your boiler pressure vessel is faulty. If you call a heating engineer out to investigate, ask him to test the internal pressure vessel to make sure that its still pressurised, he can do this with a decent foot pump. If the internal vessel is not holding pressure then the cheapest option is to have a new external vessel fitted to a return pipe on your existing pipework.

Hope this helps,

Tony
Warm Zone Heating & Solar

You put the preasure in and it excapes daily-Just because you cant find where the preasure is excaping does not alter the fact that it is excaping.
So taking into consideration the above facts, what is your question?

There is a leak somewhere.You have a pressurised system,the only way the pressure can drop is through a leak.Just because you cant see it doesnt mean it isnt there