We've experienced that kind of problem (pressure going down) over the past 2 years and couldn't track down the cause. Some months ago - the plumber (a different one that time) had 'bled' most of the radiators, and then the problem recurred. Getting fed up of going to the airing-press with the cylinder/gauge to 'top up'. If that is due to leak - how can 'invisible' leaks (e.g. underground outside the house, to the boiler/burner, piping concealed inside the internal walls) be traced? As ECDublin said, there's a real need for some kind of device to track such leaks. any ideas?
allendog