Is this any use at all?

You probably have gravity circulation on your hot water cylinder. Adding the
thermostat to it may actually be quite difficult. This is because it will
probably need an additional zone valve and wiring between the zone valve and
the programmer. This is assuming the feed/vent arrangements are suitable for
the insertion of the valve. You may want to wait until the back boiler
expires, unless you are a "have a go" type. Let me know if you are!

Christian.