I know it should only be a quick job and more accessible FACT, but I've done the PRV and also the ex vessel numerous times on greenstars and to remove the whole boiler is quicker IMO. Drain down and isolate valves, Condense disconnected, flue disconnected, ex vessel disconnected, five nuts opened boiler off. If you remove the block there's more 'O' rings, washers and seals to replace than removing the boiler.