I just wanted to say thanks for everyone's help in teaching me one or two things about which I was clueless before.
In summary, I was able temporarily work around failure of the central heating by manually opening the radiator circuit valve (which was broken) on the front of the water tank, along with setting the programmer to activate the hot water, which was the only way of running the system pump and firing up the boiler. Replacing the radiator motorised valve (not done by me) did fix the problem so that now the pump runs and the boiler starts when the programmer turns on central heating.
However, why the mains-powered room thermostat was previously clicking at the right dialled temperature to call for heating, but had stopped when I noticed this whole central heating problem, is not clear to me. In fact, the thermostat still doesn't click in any position but the central heating comes on only when the programmer enables C.H. regardless of the room thermostat, which unfortunately now appears to be redundant (or maybe the inside temperature at the bottom of the house is currently below the minimum 10 degs setting, which I find hard to believe, but I don't have a thermometer to be able to check).
In summary, I was able temporarily work around failure of the central heating by manually opening the radiator circuit valve (which was broken) on the front of the water tank, along with setting the programmer to activate the hot water, which was the only way of running the system pump and firing up the boiler. Replacing the radiator motorised valve (not done by me) did fix the problem so that now the pump runs and the boiler starts when the programmer turns on central heating.
However, why the mains-powered room thermostat was previously clicking at the right dialled temperature to call for heating, but had stopped when I noticed this whole central heating problem, is not clear to me. In fact, the thermostat still doesn't click in any position but the central heating comes on only when the programmer enables C.H. regardless of the room thermostat, which unfortunately now appears to be redundant (or maybe the inside temperature at the bottom of the house is currently below the minimum 10 degs setting, which I find hard to believe, but I don't have a thermometer to be able to check).