I have a three year old combi boiler system. Recently I have had to
refill the boiler using the filling loop quite frequently (weekly..),
because the pressure has dropped below 0.5bar (and the boiler stops)

If it is a fault with the boiler, I will not be able to do anything to
it myself, and will have to call out a plumber.

But is there anything I can do to try and work out what the problem is
before I call him?
I cannot find any leaking radiators (I assume this could cause the
symptoms). And it does not appear to be 'venting' excess water out of
the 'overflow' pipe (difficult to tell because of recent wet weather

So where could the water I am adding be going?