Depends what you want the thermostat to do, likely the Nest e is best option for DIY fitting, personally I prefer not to rely on batteries, and the comparison chart does show some advantages with Gen 3 over e. However Nest is also OpenTherm enabled (both versions) and seem to remember Valiant boilers are also OpenTherm enabled?
Having got it wrong, and tried to get better control with wall mounted thermostats, I now realise my errors, main point is the cold wall, in theory thermostat should be on opposite wall to radiator facing radiator at 1200 mm from the floor, distance from floor is because it is both manually altered and needs one to view it's display not anything to do with where it works best it is one of the Parts of UK building regulations.
But as well as cold wall there is the time taken for heat to reach the thermostat, using modulating (analogue) controls like OpenTherm helps, so no on/off it instead sets boiler output up/down to suit.
But what transformed my mothers house was to actually set the thermostatic radiator valve (TRV) and not try to use a single wall mounted thermostat to control all rooms, I selected Energenie MiHome as I also wanted some wifi sockets and they use same hub, I had intended to add Nest latter and I knew Nest was suppose to work with Enegenie MiHome TRV heads, but in that house never installed Nest. Reason for not installing Nest, was the TRV heads controlled house A1 so no need for Nest.
On moving to this house, there was not wall thermostat, and there was also some wiring faults, so since already had the TRV heads from last house, Nest Gen3 was a clear selection for a wall thermostat. However it does not work well with Energenie MiHome TRV heads, yes on the mihome app if I adjust Nest then the TRV heads follow, but with schedule changes they often failed to follow, what the should do, is not what they did do.
Nest works well, and the MiHome TRV heads work well, but they don't work together as they should. I have also read that Hive has the same problem with their TRV heads not communicating with the wall thermostat as advertised. I suppose no perfect system, Tado will not release info on how it works, EvoHome needs an add on module to work with OpenTherm, and many ebus connected thermostats will only control one room.
However often that is all that is needed, control one room A1 and rest of house relies on the TRV heads, as long as that one room needs more heating time than any other room, it will work, but that means in the main the house is controlled by TRV heads, not the wall thermostat, and as I say we learn from errors.
In this house, with oil fired boiler the pre-wired point for the thermostat is not opposite the radiator but along the same wall, although Nest works out time to heat a room so shows on changing settings the time to reach new heat, in this house it is between doors to other rooms so door left open or closed alters how well it works, so in the evening drops to 17°C over night, but in morning the rise to 20.5°C is in 4 hourly stages of 0.5°C per hour to stop the hysteresis as it over shoots when doors are left in a different position to what thermostat has learnt.
In hind sight, and hind sight is easy, wish I had fitted EvoHome, but by time I bought this house already had 4 MiHome heads, so kept to same system.
