Central heating zonal control fault

[stem]

I replace the thermostat wheel with digital timer and put in 3 core and e to the mvu will I be able to get the annex stat to fire the heating separate to the main house.

If you wire up the microswitch in the annex valve to switch the boiler on then maybe, but only if the plumbing is correct.

The first thing to check is where the annex radiator pipes originate from. If they are simply connected to the main house radiator circuit then they will only ever heat up when the main house heating is on.....That's a restriction of the plumbing.

If the fault has always been there, it is more likely that when the system was extended, the new radiators were tapped into the pipe feeding the existing radiators nearby because it's easier to do, instead of being connected back to the start of the hot water distribution (usually just after the pump). Then a motorised valve is added to the branch going out to the extension, and a thermostat fitted to that. But the main house heating also has to be operating because the extension is connected to it.