Losing water?

Hi,

I have a Vaillant eco TEC 831 boiler. It has needed topping up several times in the 3 years since it was fitted. This weekend, it needed doing again. While filling it, I noticed the carpet was damp under the boiler. Then I noticed that it was dripping from the bottom almost as fast as it was going in. I can get the boiler to fire up and run for a couple of hours until the pressure drops enough to knock the boiler off.
Can anyone tell me what could have gone wrong as my plumber is a bit suspect and I have already caught him out trying to fleece me once. This has given me a deep mistrust of all plumbers. Forewarned is fore-armed as they say.

**** I do not intend attempting a repair myself, I just want some idea of the problem.