In message , SantaUK writes
I been thinking about doing this myself and the simplest way would be to use an additional thermostat on the hot water tank set to the lower (or higher) temperature, it could be positioned at a height as well to reduce the volume of heated water( rather like having two immersion heaters on economy seven). the only problem is you need a two-pole time switch if you got a change-over type of thermostat like Y-plan ones. I suppose if you were really clever you could ad a set-back heating element to the thermostat, but that's not really a DIY proposition!