Step one
read the instructions for boiler which on page 23 shows volt free on/off control, I have looked for the opentherm connection and as far as I can see the boiler does not have the option of an external control page 30 confirms it is a modulating boiler, but like many it seems it can only be modulated by the return water temperature.
I am an electrician not a heating engineer, however it does seem modulating the boiler output is important, and with your boiler the method used is the thermostatic radiator valve (TRV) you can swap the heads on the TRV to electronic type, and Nest is designed to work with MiHome Energenie heads, it uses "If this then that" (IFTTT) to use a follow command so either the heads follow the thermostat or the thermostat follows the heads.
I fitted two heads with the idea of then fitting Nest, however once the heads were fitted I found did not really need Nest, and also found my boiler needs a Wave thermostat it seems it is not designed to work with opentherm.
What I have worked out is a boiler works better by modulating the flame height than by simply switching on/off, however unless you use some thing like EvoHome then the problem is to tell the boiler when to switch on, switching off is the easy bit, as the water temperature rises the flame height reduces, once it can't reduce any more then it uses a mark/space ratio of switching on/off to further reduce output, it would be possible to set the boiler to completely switch off when the space exceeds a set size although this is not done, but there is nothing to tell the boiler when to start again, as once water stops flowing there is nothing to tell boiler if required.
Personally I manually switch it off in the summer, but having a thermostat to automatically switch the heating on when required is an advantage, but you don't want the heating on when likely to be a warm day, so around 18°C would be a nice point to auto switch it on, however 18°C is a bit cool in the winter, so the thermostat has to be located in a room normally kept on the cool side, with no outside doors and no heat producing items in the room, you may have such a room in a mansion, but in the normal house nearest thing is the hall, which has an outside door.
I have got around the problem by using two thermostats, one in kitchen and one in hall, and fitting a TRV in the hall, which all the instructions say don't do, but it works, however it relies on not changing the temperature, as the TRV temperature and wall thermostat temperature are matched, so change one and the other needs adjusting to match.
Since the TRV heads allow geofencing I thought I would try it, set them so when 30 miles from home heating turns on, it was a failure, what I realised is the heads use a gradual turning on and off of the water, so it will keep the room at spot on 21°C as set, but if set to 16°C then back to 21°C it will warm from 16°C to 19°C quickly, but slows down then so as not to over shoot so two hours latter before sitting at 21°C, and that is when I fiddle the thermostat to keep boiler running. Set to 25°C and 21°C is hit quite fast, so not the boiler, it is the control which is the problem.
I quickly realised the only way was using time of day, so switch to 16°C at night, then 24°C at 6 am and back down to 21°C at 8 am and it works spot on, this means the whole idea of controlling with phone rather pointless, if you have to take out your phone to manually change rather defeating the whole idea of automation and you will forget, and geofencing would need you to work 3 hours from home.
However I did try a cheap programmable thermostat, around £80 with wireless connection, when it went wrong it did not fail safe, the house either got far too hot or far too cold, because Nest has two way coms then if coms is lost it fails safe. However in my other house the use of a hard wired programmable thermostat with a non modulating boiler has worked well. In that house the TRV stops bedrooms over heating, but down stairs there is just one thermostat in an open plan house.
So back to your boiler it seems there is both a no volt and a mains voltage option. I would use the volt free option. It would seem 2 and 3 on Nest replaces the link on the boiler. But consider where to fit and if TRV heads will be added latter, clearly if a TRV head is to be added to living room radiator then the Nest thermostat should not be in that room.
