As the new SCR hasn't solved the problem, you should go back to Drayton. It's their problem, after all, not yours. Trade Description and Sale of Goods Act etc (not fit for purpose and not of merchantable quality).
I did go back to them, but I haven't got any further response (yet). I don't think I can invoke the Trade Description and Sale of Goods Act as this was installed four or five years ago before we moved in. So although I agree that it is not fit for purpose, it is likely too late and I'm not the actual purchaser. It's interesting that my diagnosis was better than theirs. I did ask them how long is the id sent by the thermostats but they didn't answer that. I thought that if the new SCR had fixed the situation then it could have been that the logic that stores the the id at pairing time and then compares the stored id with the received id could have improved in more recent version of the SCR. It would be nice to know the protocol they use and how long is the transmitted thermostat identification (and if all the transmitted bits are used by the receiving unit), but they don't seem willing to share that information.
Does your boiler have a Frost Stat ? If it does could that be switching the boiler on.
Yes I think it does. Theres a rotary switch that increases the water temperature for the radiators, and it goes below minimum to a frost position. I assume that's what you call a Frost Stat. I'm not sure exactly how the Worcester combi boiler interacts with the thermostat when put into this frost setting, but I intend to set it that way when we go away in the hope that it will help both avoid the flat going too cold and getting a too high gas bill for when we'll be away