How do I fix a Baxi Bahama thats losing pressure FAST!?

Just bought a house thats 4 years old with a Baxi Bahama Boiler.
Its been serviced every year by the previous owner (proof seen!) but now the autumn has set in and the CH needs to be on I've noticed that the water pressure is plummetting from 1 bar to almost nothing within 24hrs (particulary when heating is off, which I don't understand.)
I've checked all the visible pipes and joins for leaks and can't see anything. Any ideas whats going wrong?
I'm waiting for my British Gas CH Care to come through do you think if I can wait they'll fix it?

Help Please!