Boiler not responding to external wired switch
I managed to isolate the fault down to the RF receivers. Both on the blink. Receiving ON signal but not opening the switches to their respective valves.
To do this I uninstalled both receivers and manually opened the valves, such that I had a basic setup of just the main time switch and no thermostat. Everything working.
Then I took one of the RF receivers and installed it in place of the time switch, using its remote as my room thermostat and timer. Intermittently the receiver failed to open its switch to the boiler. Giving the unit a whack tended to fix it!
So now I know which part was faulty.