Hopeless, I had texted Matt the model # of my gas hot water system and suggested it was probably the diaphragm - the most common part to wear out over time in constant flow Gas water-heater systems. His guy turned up with no parts, did some unnecessary tests for 40mins ( gas & water pressure - which I knew were ok) then suggested it "might be the diaphragm", which he would need to order in and then return and install. I then rang Ray Smith(Complete Gas Services) his guy turned up with parts, replaced the diaphragm - fixed in 20mins.