Stuart 504 spirit & gas-fired boiler

A Stuart 504 boiler, largest of the 500 range, complete with original twin leg spirit burner, although now equipped with a gas burner - a much more civilised arrangement, allowing fine control of the flame. Problem with the spirit burners is that the boiler quickly comes up to working pressure, then sits with the safety valve blowing for half an hour while you're trying to have a quiet run with a beam engine.