Hi again Rab
Info from A N Other forum suggests you wish to have a thermostatic switch to control your centarl heating pump. You wish to be able to get the water temperature up above 40 degrees and to also prevent overheating of the water jacket.
I think mick described the orange thing in my photograph on The Other Forum which is basically a bi-metal strip in a plastic case. The Bi-metal switch is temperature activated and turns the pump on when the desired water temperature is achieved and turns it off when the water temperature drops.
If, like us, during the night in the depths of winter proper and you've banked the fire up, closed the dampers and tried to ensure the fire stays alight all night, if you want the system to pump a bit of warm water round the house every now and then, the only thing you need is the orange thing.
Broseley, a well known wood burner supplier in UK obviously know what they're about, however, having used woodburners for years (before we came to BG), sulphuric acid corrosion didn't figure in keeping the system optimum. De-sludging and flushing every few years seemed to be enough.
Then again, this is BG and therefore is bound to be different.