We had a fault with ours where a dry joint in a circuit board on the boiler meant the pump wasn't 'called on' when either CH or HW was demanded.
Very quickly indeed the water in the HW circuit (ie indirect coil in the copper cylinder) boiled and the boiler overheat operated.
I'd expect Sooty's to do the same if the pump is borked unless it's a direct thermosyphon.
My guess is a switching issue on the CH side.
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