Thanks for the reply - it does seem like it is overheating but the thermostat cuts in and if on water the gas valve is not opened, if on rads only then the boiler sometimes does not restart - ignition fires (you can hear the piezo clicking) but there is insufficient gas - no circuit made on the...
I have a five year old Sime boiler - not sure on the model Super 4000 or something - If it is set to CH only it generally works OK but occassionally stops heating and won't come on until boiler is turned off cools and starts again. DHW works initially but flame failure occurs after first...