Having looked into this recently, the cable connecting the heatlink to the thermostat doesn't need to have an earth (There is no earth terminal on the thermostat base!)
Correct however BS 7671:2008 says "A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point." but I have never run an earth with a separated extra low voltage supply, i.e. door bell, or fitted a RCD and there is nothing in the BS7671:2008 that says the RCD requirement is only for low voltage.
So common sense says no need for an earth, which is why I said technically it needs an earth as Nest says one should be fitted when hard wiring the thermostat, but see no reason for it.
More of an issue is the inability of Nest to work with electronic TRV heads. Basic idea is a wall thermostat in a room kept cool, so when summer arrives room is warm enough to not switch on heating, on the lower floor and heat raises, with no alternative heating and no outside doors. Don't know about you but with my house no such room.
My first house was hot air gas central heating, ducts to each room, and vents in doors for air to return, and a single wall thermostat worked fine as air circulated. Second house open plan, so TRV's upstairs to stop bedrooms over heating, but Myson fan assisted radiator ensured again air circulated so again one wall thermostat was enough.
It was only when I moved in with mother to look after her, that I realised the problem with one thermostat down stairs. She had bay windows facing east and west, and if the sun came out then front and rear of house were heated alternately as the sun moved around during the day, so it was important that the TRV heads controlled room temperature not a central thermostat, and the radiator was just warm enough, no more, so when sun came out it could cool quickly.
I fitted two thermostats in parallel one in kitchen and one in hall, and also a TRV in the hall to speed recovery when front door opened. And electronic TRV heads in most rooms, which were take to this house when that house was sold.
This house has two wall thermostats and two motorised valves, and the flat under the house has one thermostat and the main house the other which is Nest. But main house the problem is not heating but cooling. Every down stairs room has outside doors, we have four doors to go outside, living room has an open fire, and kitchen clearly we cook in, so hall seemed the best location, but hall is centre of house and last place to cool, so any over shoot and rest of house gets cool before boiler switches on again.
But the point is every home is different, and there is no one system fits all. And it seems Worcester Bosch has not signed up to OpenTherm so either you use their own thermostat which can modulate boiler output, or a simple on/off thermostat. If the home is split into two sections there are thermostats designed for this, Drayton Wiser does one, but likely any simple on/off thermostat will do.
I fitted Nest Gen 3, and not impressed. In hind sight even a cheap Hive would have been better. But it was fitted so the heating did not rely on batteries to work, or RF, being a radio ham relying on an RF link is not good. And it is hard wired and it works of sorts, good enough not to be worth replacing.
