Leaking Ariston Boiler?

I have an Ariston Gas Boiler and it has the 3 zone heating area, downstairs, upstairs and the water. The problem is that is is losing water power every 1-2 nights from 2bar to 0.3bar. Some mornings I wake up to no hot water as the water pressure was too low to kick start the gas boiler. I have had 4 plumbers look at it on different occasions and they all sealed up little parts here and there but no permanent fix can be found. Is it possible that I have a major leak somewhere under the floor? I have to top up the water nearly everyday. Please help me!