Your installer is at fault for not checking the service pipe was of sufficient capacity. Personally I would never install a combi of this type and Kw capacity. A house of your size commands a conventional system ie a heat only boiler coupled with a cylinder (unvented-provided water supply pressure/flowrate is adequate) with the all important immersion heater backup when the boiler packs up.
Transco are not required to upgrade the network to cope with these oversized combis...if they are doing it for free you are very lucky. Remember it is common to find 2 or 3 mbar drop from meter the boiler gas valve test point despite correct pipe sizing after the meter...Vaillant have issued guidance on this. Legally Transco only have to provide 14 mbar (or thereabouts). New governors are sealed and normally setup correctly. However, there is no reason it could not have been adjusted but this is probably futile when the service is undersized.
It's your choice but given the outlay so far, replacement service pipe is probably the simplest route. Just make sure Transco guarantee the district pressure (ie the main gas pipe in the road) is sufficient (some networks are very undersized due to the shift over to combi boilers).
Unfortunately heating and hot water is taken for granted...homeowners don't want to spend the extra for a correct and sensible system (ie seperate boiler/cylinder), many installers lack the knowledge to install them, and the network operators are only interested in operating to the legal minimum standards.
Transco are not required to upgrade the network to cope with these oversized combis...if they are doing it for free you are very lucky. Remember it is common to find 2 or 3 mbar drop from meter the boiler gas valve test point despite correct pipe sizing after the meter...Vaillant have issued guidance on this. Legally Transco only have to provide 14 mbar (or thereabouts). New governors are sealed and normally setup correctly. However, there is no reason it could not have been adjusted but this is probably futile when the service is undersized.
It's your choice but given the outlay so far, replacement service pipe is probably the simplest route. Just make sure Transco guarantee the district pressure (ie the main gas pipe in the road) is sufficient (some networks are very undersized due to the shift over to combi boilers).
Unfortunately heating and hot water is taken for granted...homeowners don't want to spend the extra for a correct and sensible system (ie seperate boiler/cylinder), many installers lack the knowledge to install them, and the network operators are only interested in operating to the legal minimum standards.