@EFLImpudence has given you old wiring, however the old thermostat will work without a neutral and also with the red and yellow crossed. May not work as well but it will work.
I assume it is the MoesGo thermostat at around £40 which seem popular, but to work with battery and no neutral with Smart capability looking at the Nest E at around £140, for £100 worth testing neutral and seeing if it can be fixed.
I am not sold on the smart thermostat yet, to be able to use geofencing you need fast heat up times. And so over sized radiators for the rooms, and a sequence to ensure first room likely to be used is first room heated. It seems Drayton Wiser can do this, with both the TRV and the wall thermostat linked using geofencing on both, although never tried it.
However I have tried geofencing with both TRV and wall thermostat, in my case Energenie and Nest Gen 3, and the problem was the Energenie TRV, it had an OTT anti-hysteresis software built in, the result was it could take 3 hours to reach set temperature, the Nest Gen 3 was better, but with lock down not really tested, however with the 3 hour time, in real terms simple programmable wall thermostats and TRV heads seem in the main to work better, old house had simple programmable wall thermostat battery powered, so just two wires, and the eQ-3 TRV heads in this house have worked well, still some problems but in the main works well.
The problem is radiator size, to set it to heat kitchen, dinning room, living room, bedrooms with a delay between each so rooms are heated as required means radiators need to be over sized, so I just heat whole house together, but to test how it works would need to give wife my phone and not go into the hall, and see if boiler fires up while she is away. Not done that so really not a clue if it turns off or not when I am away from house.