My Main 30HE loses pressure over 24 hour period?

Is it a combi boiler? If so there is an expansion vessel that absorbs the expansion of the water in a closed system. You need to pump this up so that it absorbs the expansion. If this is what it is you will find that it starts with pressure, the radiators heat up and when they cool the pressure drops to zero. Otherwise it may be a different problem

eduk
October 2013