You can see the screws here, seem to remember to one side.
As to fitting Hive, Nest, EvoHome or any other control system step one is work out what you want it to do, non are perfect, every one has faults some where, so it is a compromise.
So there are three ways to control the boiler output.
1) An on/off switch that turns boiler on or off as required, the problem is this can upset the boilers built in controls if turned off/on too frequent.
2) Turn the boiler up/down by connection to the ebus, Opentherm is most popular.
3) Use the return water temperature to turn the boiler up/down.
In real terms we often use a combination of two methods.
So there are two thermostats used to measure room temperature and control it, one fitted on the wall and one fitted to the end of the radiator, these can be independent or linked, Hive is an odd system out, most the better systems connect to ebus, but Hive allows the return water to control boiler output by acting as a hub taking a demand for heat from the Hive TRV heads, the idea seems good, but it clearly only works when you also have at least some Hive TRV heads.
So I would say step one is look at what you want to spend, and decide then what combination to use, I will admit I got it wrong, so I am admitting my mistake in the hope you don't make the same mistake. I bought first two then another two energenie mihome TRV heads, they worked very well once set up, and the four rooms with them fitted stayed at the set temperature without a problem, they were designed to work with Nest, and when I moved home I took them with me, and added Nest, this is where it all went wrong, it seems Nest had withdrawn support and they no longer linked together, I also bought 5 eQ-3 TRV heads bluetooth much cheaper, and they work just as good as the expensive mihome heads, yes they are not wifi so I can't set them to work with geofencing, but I found simple programming of the heads worked nearly as well as using geofencing.
This idea of as you drive home it sees you phone is getting closer so turns on the central heating is great, but for a TRV not to over shoot it has to raise the temperature slowly, so at 6:30 am it goes from 16°C to 20°C and very fast it hits 18°C, but to hit 20°C takes 3 hours, so I set at 6:30 to got to 22°C then at 7:30 am to 20°C and then it worked A1. But that does not work with geofencing it only works with simple programming. The Danfoss I think are claimed to work out how long it takes to heat the room and so work faster, but it is the TRV that is all important not the wall thermostat, it does not matter if the Nest wall thermostat works out how long it takes to heat a room, when it is the TRV which is controlling it.
So a eQ-3 costs around £10 without blue tooth and £15 with blue tooth the terrier i30 is about the same the way it works, but the mihome TRV head is around £35 each about the cheapest with wifi, some are up to £60 each, so it's not the cost of the wall thermostat it is the cost of the TRV heads that matter.
So you need to look at whole cost, be it 3 or 30 TRV heads plus a hub/wall thermostat, not just the wall thermostat on it's own, if I was doing it again it would be all eQ-3 TRV heads and a very cheap wall thermostat, all the wall thermostat does is turn off the boiler on warm days to stop it cycling, it is the TRV's that do all the work.
