Hmm.
It *could*, and it's a very slim *could*, be a boiler fault.
Pressurise the boiler to 2 bar, then isolate it using the valves beneath the boiler. YOU WILL NOT BE ABLE TO USE THE BOILER LIKE THIS so turn it off on the mains power to prevent anyone accidentally firing it for hot water.
If you then leave it like this overnight, check the pressure in the morning. If it has dropped, you have a fault on the boiler - possibly a hairline crack in the main heat cell (which may be covered under warranty). You may also notice water coming out of the condense drain even though the boiler's off.
If the pressure stayed the same, you have a leak somewhere on your system. Prove it by opening up the isolation valves again, if the pressure then drops it's a dead cert that there's a system leak somewhere.
BunSlinger
September 2008