Its your diverter valve, I had a similar issue with one recently, exactly like your symptoms, and all it needed was removing the diverter valve, and it can be serviced as the new ones are quiteb expensive, the fact I didn't even put any new parts or rubbers, just had it thoroughly cleaned and re-greased with silicon grease and re-assembled it and all went fine, however, mine was a Biasi H24 boiler, its the non-condensing boiler.
