Many thanks for all your replies. It is a 1960’s gas boiler, all the jets are clean and working and the flames have their usual appearance. The heat exchanger fins etc have been recently cleaned. The boiler thermostat is working but I have to turn it quite low before it switches burner off, at this point I can hear the pump running. I have now opened the boiler drain and after initial splurge of black and spurting across our breakfast room carpet (fortunately managed to clean carpet with soap and water before her indoors saw it!) the water appears clean but does actually have a few black particles in it. Basically I see it as a kettle full of water, switched on, but not heating the water as it should!. There are no electric valves in the system. I am now going to run it with pump not running ie gravity feed to hot water storage tank, towel rail in bathroom and one radiator only and see if boiler gets up to temperature?