The Horstmann connections are:
All of the existing wires to the Horstmann terminals relocate to the Hive receiver terminals (you will require the dual channel version) that have exactly the same function.
The exception is the live link wires shown dotted between L / 2 / 5. The Hive Dual Channel has these made for you internally already, so they can be discarded.
N to N
L to L
Earth to Earth
The black wire in Horstmann terminal (1) called 'HW on' would go to the Hive terminal (3) 'Hot water on'
The brown wire in Horstmann terminal (4) called 'CH on' would go to the Hive terminal (4) 'Heating on'
Because the Hive carries out the functions of the room thermostat, the old wired thermostat needs to be decommissioned to prevent it overriding the Hive. It can't simply be disconnected as this would leave the heating wiring 'open circuit' and it will not operate. The thermostat can be decommissioned in several ways.
1. Find the origin of the thermostat cable. Note where the Brown and Black wires are connected and disconnect the entire cable. Then link the terminals where the Brown and Black wires have just been removed from together so that they are electrically connected. The old thermostat and its cable can now be removed.
2. Remove the existing thermostat and replace it with a junction box, connect the Brown and Black wires together and isolate the Grey neutral and earth
3. If you are happy to leave the current room thermostat in place and simply disable it, leave it where it is, and put the black wire in the L terminal along with the Brown wire that is already there.
You don't give much detail of the heating system the controls are connected to and it's a bit unusual (but not unknown) to have a combi boiler connected to a hot water tank (cylinder?) But if your system is fully pumped that is the default setting for the Hive, if it has pumped heating with gravity hot water circulation, then the Hive should be set to 'Gravity Mode'