now i am gonna sound dense... but where might the hot water stat be?

It should be a little box near the bottom of the hot water cylinder. It may
have a knob on it, calibrated in celcius. It will have a flex coming out
wired into a nearby junction box. It may be held on with a plastic strap
that goes all round the cylinder. You may confuse it with an immersion
heater, if you don't know what you are looking for.

You may not have one, in which cause, you should fit one and use it to
control the zoning arrangements. Otherwise, you are throwing money away.

Christian.