Sounds like the gas regulator/convertor. Call TVH and ask for a Woodward N2001 (Crown part XA343007), you don't want to know the Crown price believe me! Unfortunately you probably need all the relevant fittings for the gas hose outlet connection, coolant hose connections and vacuum hose too, you can probably source those locally, theyre just brass fittings. The coolant thermostatic valve TVH should be able to supply although the unit will run without it. Oh, there is one available on Ebay...take a look.