I look at the picture of inside your boiler and question why there is no loops on the room stat /timer, heating zone 2, and frost stat? I would have expected to see a loop of wire which needs removing at least on one of those.
I also question use of Hive to start with, if using the thermostat in the traditional manor installed in the coldest ground floor room without alternative heating and with not doors to outside then the thermostat needs to be one without any anti-hysteresis software, as you don't want it to switch off/on all the time, every time it switches off energy goes out of the flue.
If however you are using the thermostat to control the main living room, then it should be of the opentherm type.
Hive has anti-hysteresis software built in so not suitable for use in coldest room, and is not opentherm so not suitable for main room.
There are boilers, like unfortunately the one I have, where the control can only be with the return water, there is no option for any modulating thermostat to be fitted other than the one built into the radiator valve, there are others which can't modulate old gas or oil, and with oil some where the modulation is that little may as well consider it as not a modulating boiler, so in my case only option for better control is electronic heads on the thermostatic radiator valves (TRV) even with boilers with opentherm or other modulating option often we want to control rooms independently, so still the eTRV is best option.
I just can't see how Hive fits in with a boiler with opentherm option, Nest or Tado yes, Hive is OK for oil and very old gas boilers, but few modern gas boilers can really be controlled in an economic way with Hive.
However I would want to read the instructions for your boiler, and work out why there is no loops in the room stat /timer? Maybe there is a dip switch, or it needs a computer plugging in and configuring? Normal is you remove loops to fit room stat /timer to fit an external timer.
