In reply to balmybaldwin:
> (In reply to Cthulhu)
>
> Thank you, my next question then:
>
> Is the system safe to leave on for HW until I have time to get this sorted? (as it's warmed up I can live without heating for a few days)
Yes. Or, there's a little lever on the 3-port valve that you can move across to the heating position. That'll keep the valve in the mid position, so water and heating have equal priority.
>
> Also, how do I test that it's the valve rather than the HW tank stat (so I don't buy one unnecessarily?)
You can bridge the wires on the stat, which'll complete the circuit and send the satisfied signal.