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?