The message

from decker contains these words:

We have a standard Gravity Fed System Setup
Potterton Profile (in garage)
Electronic Timer (in kitchen)
Thermostat (in Hall)
Zone Valve which switches between water and Heating (in airing
cupboard by water tank).

Stuart

Couple of stray points.

You don't mention it but I presume you do have a thermostat on the hot
water tank.

What is the electronic timer? That might have some bearing on the
situation (but on the other hand might not).

--
Roger Chapman