OK so you have a Combi boiler. They don't normally have a separate hot water cylinder, which the dual channel Hive is designed for. So, either.
1) You don't have a hot water cylinder and have purchased a dual channel version by mistake, or
2) You do have a hot water cylinder connected to your combi, in which case the Hive won't be wired directly to the boiler, it will replace the existing programmer.
I'm assuming because you don't mention a hot water cylinder, or programmer that 1) is the correct scenario. If it's 2) then please post back as the instructions below will be incorrect.
First of all, I need to point out that if you have to remove a room sealed enclosure to gain access your boilers wiring terminals (some you do, some you don't, and unfortunately I don't know which category yours is) then it is not a DIY job. It should be done by a Gas Safe engineer who can make sure that it is correctly sealed up again afterwards.
You really have the wrong version of Hive. The Single Channel version is better suited to a combi. Some combi's can't be connected to a Dual Channel version because they only have 24V thermostat circuits and the Dual Channel can only provide 230V control.
However, your boiler does have 230V control, so the Dual Channel can be fitted, however the hot water functions whist present at the Hive, won't actually do anything. So personally, if it were me, I would exchange it for the Single Channel version, that doesn't include the separate hot water control.
View attachment 154375
The 230V supply for the Hive (N) & (L) comes from the boiler (N) & (L) terminals included in the group marked with the 'F'
Hive terminal (4) 'Heating on' connects to the terminal marked (1) included in the boiler terminals marked with the 'F'
I assume by "external switch" you are referring to the existing room thermostat, that should be disconnected. The jumper 'D' is not required and will have already been removed when the existing external controls were added.