Interesting responses thank you. Giving Aga a call sounds a great idea and I
will do that. The pumped system will need to be turned right down so as not
to exceed the modest heat output capability of the Aga which is oil by the
way. I don't really want to run a 240,000 BTU boiler just to heat a water
cylinder in summer. The boiler will feed something like 28 radiators so the
prospect of all being shut down sounds unlikely - a no-flow switch closing
down the boiler sounds much more sensible than a room thermostat in one
room.