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thanks to all contributors on this one. I'm replying to rob884's question about the cycling on and off rapidly, because it does tend to do that. As if it is hesitating and can't make its mind up - a succession of clicks before it comes on or decides to stay off. As to precise model: it is a...

Halloa again. There isn't unfortuantely. But it isn't the cyclinder that has the water too hot. It is the pipe that goes through the boiler. That seems to get too hot too quickly while the cyclinder is still heating up. And that makes it go up the overflow instead

Sorry again for inprecision: the thermostat on the boiler. There is no independent thermostat on the tank.

Thanks for that - very prompt. The first thing (I forgot to say) is that on domestic it is gravity only - the pump only coming in for the rads. (I know they say all pumped is more efficient, but it worked Ok like that for 8 years.) Parts replaced were potentiometer, thermostat and overheat...

After years of perfect operation the boiler has starter (intermittently) to overheat the water (when on domestic only) making it boil up and overflow into header tank. Cured by turing off (at timer) and straight back on. Thermostat replaced, system flushed out, Plumber baffled. Any ideas?