I have that feeling I have seen this post before? It really should be a new thread, as answers to one persons question may not be valid to another persons question, the first thing when changing a thermostat is to find out what type is required, there are three basic ways for a thermostat to connect to a boiler, 230 volt on/off, 24 volt on/off and variable control to the ebus, there may also be some other odd methods, but those three are the most common.
So next is if the boiler can modulate? if it can modulate can you use a modulating thermostat, or can it only be controlled by return water temperature? Then you look at premises, how do you want to control it, with open plan 1980's houses a single central thermostat is likely best, but with houses with doors, likely you want TRV control or fan assisted radiators.
You need to weigh up what you have and decide how to improve, and with a modulating boiler where there is no access to the ebus then likely Hive is a good option, but Hive duel channel is 230 volt control only so depends on the boiler, Hive single channel is volt free.
So Hive single channel receiver needs 4 wires, there will be 5 wires as there is an earth, but that is only parked for future use, you can reduce the wires by one if the control is 230 volt, it is likely it is 230 volt in
@Hamilton1985 case as the existing thermostat likely has a compensating resistor which needs 230 volt, but can't simply assume that.
So terminals L, N, 1, and 3 are used with Hive single channel, there may be a link L to 1. But if I try to go further it may confuse the originally poster so best is start a new thread and if there is a wiring diagram on the original programmer or thermostat give a picture showing it, and a picture of the wiring and boiler type or anything that tells us if mains or reduced voltage used to control it. In some cases instructions don't say, so then down to measuring with a meter.