Thanks Andy and flameport. I have a central heating electrician coming round tomorrow who is a mate of my brother in law who is a sparks.
The point you make about one of the 2 wires not currently being used makes a lot of sense. Hopefully the guys tomorrow will know which one. I will keep you...
I have just replaced a very old room stat which had 2 red, 2 black and an earth. Drayton technical told me the wires to use from the old stat which involved taping off two of the wires as only two are needed for the new stat which are common and on.
When set temp calls for heat, boiler fires...