Paloma Bob

Hi. I’m hoping you can help me. I have a Paloma PH5-3F and the gas burners aren’t coming on to heat the water. The temp adjustment valve is ceased as well, but this didn’t effect the burners until now. I have dismantled it from the heater and tried descaling, also tried WD40. I have replaced the diaphragm, but nothing’s working. There is good water flow and the pilot lights no problem. When I turn the tap off quickly the burners light open for a split second and then go out. If I turn the tap off slowly they don’t light up. I thought maybe trying this a few times would free things up, but it’s not improving. 

 

Any sensible suggestions on what to try next very much appreciated