If I want to modify central heating then step one is find out what I already have.
the instructions on page 23 seem to me to show the boiler has a "BUS" connection so I would assume any thermostat would be of a type that connects to the bus?
I am not a heating engineer, just an electrician, however I understand for wall thermostats there are two very different types, the old digital (off/on) and the new analogue (modulates the boilers output) and often boilers can use either, also with the digital you have mains versions, and volt free and it would seem if using the old type it needs to be volt free with your boiler.
As far as I am aware even with boilers that can use modulating thermostats, they can still have the old type fitted, however the job the thermostat does is not the same, analogue thermostats control boiler output so are either part of a hub or in the main room, digital thermostats stop boiler cycling during warm days as summer approaches so would be more likely placed in the hall.
I am also confused as to where and how wall thermostats work, clearly room temperature is controlled by the thermostatic radiator valve (TRV) and with electronic types these tell a hub what output is required, which in turn tells boiler what output is required. And I see there is a problem in that unless this system is used, if return water temperature is used, then as summer approaches we can have a boiler cycling (switching off/on) just to test if hot water is required, and a wall thermostat in say the hall can switch off the boiler and stop this.
However in winter I want house at between 18°C and 22°C but in summer I want to keep house cool so 16°C to 20°C in fact 20°C is likely too hot, so can't see how we can really automate the boiler switching off, however many devices I install I still turn heating up in winter and down in summer using manual controls. To be completely automatic it would need to connect to weather reports, which can be done with IFTTT, but to my mind too complex, I simply turn hall thermostat up in winter and down in summer.
The point is before adding of changing thermostat, work out what you have and what you want, since seems your boiler modulates, temperature control really needs to be also something that gradually opens and closes or turns up and down, not off/on, however switching boiler off for summer can be off/on and I still use a thermostat to switch off/on could be swapped for a simple switch, but never bothered it is just as easy to turn a dial as to flick a switch.
I thought my TRV's were at fault because rooms were not staying steady, so I bought expensive electronic heads, then I found the problem, most of the radiators have the TRV on the return not feed, and although valve works either way being bidirectional it means radiator heats up well before the TRV gets warm, so it produced a hysteresis, simply turning down the lock shield valves which means radiator heats up slower so there was enough time for TRV to close before radiator stinking hot, and all now works as it should.