Yeah, they're funny things thermal stores. The stored water is the primary water and not what the user actually uses for DHW.
They heat the incoming main via a flat heat exchanger on some models and on the BP model (with a blending valve) it uses a coil, but in reverse if that makes sense!?
The stats that they provide don't have a thermal cut out, though I have fitted standard stats in their place and just turned them right up, only ever had one get too hot and cut out. With that one I fitted a proper factory set rod stat...cost me £50. For a rod stat! Criminal.
Anyway, I'll keep my fingers crossed for you Mike.