baxi do quite good condenser combis condensers are more efficient boilers or so they say, they re-use the flue gasses to heat a second heat exchanger they use a plastic flue because the gasses are so much cooler but the plume is like a cloud of mist so it must be 2.5 meters from a boundary line...
would check possibly a diverter or flow turbine problem it sounds like a contact which is made on your hot water is being broken could be a puntured diaphram and is getting weaker depends what is in the boiler check your manu instructions that come with the boiler
just registerd on the site seen your roblem had the same myself you will find that there is a little circuit board in the shower unit which tells the flow transducer the pressure is ok this is what has gone in your shower you will need a new one and they are quite hard to find you might have to...