Nothing comes to mind from reading the manual, Groupie. A normal combi.
There should be no direct, permanent connection with the mains water supply. It is connected of course, but only by opening two valves to top up the sealed radiator circuit.
I wonder if there's a chance that these valves are open, or maybe one or other is faulty, and overfilling the system. The pressure read-out would show this up. Excess pressure would then open the pressure relief valve and drain the excess away.
However, that all seems very unlikely.
Modern condensing combis are getting quite complicated. It's quite usual for the installer to refer immediately to the manufacturer for this kind of fault, especially if it's newly installed.
I'm sorry to be of no help, but I think this one ought to be referred to Ferroli.
http://www.ferroliuk.co.uk/