faulty boiler no one can cure worcester heatslave 15/19 combi?

my boiler is losing pressure and no one can get to the bottom of it !!
overnight the pressure drops to virtually nothing with the boiler off.
I refill it to one and a half bar then switch the boiler on at this point the pressure starts to fall and it sounds like you can hear air escaping i refill it again and it fires up, then a short while later the pressure goes up to about 3 bar on its own. the heating and water are ok and i can't find a leak anywhere.
when i switch the boiler off at night the next mornin the pressure has dropped again. Can anyone please help me !!