Glow-Worm Fuelsaver MKII

Having problems with my Glow-worm Fuelsaver MKII boiler, I know it's old I think we installed it in about 1988, but really could do with getting this winter out of the way.
The house is not getting warm & the water is not as hot as it should be. it has 2x twin port motorised valves which are opening & shutting as they should when being controlled from their respective thermostat.
The problem appears to be that the boiler is shutting down too early, the over run pump keeps pumping as the valves demand it to but the boiler doesn't continue to fire up. It seems likely that it is a thermostat problem within the boiler but can anyone help as how to proceed.
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