Hi Stu and welcome to the forum.
The EP2002 is a bit unusual because it is a Programmer and Wiring Centre combined. The terminals A B C & D are just terminals provided for convenient use by the installer. They are simply used to connect wires together and are not connected to the programmer functions. Your biggest problem is that the Hive does not have this 'junction box' facility to accommodate so many wires. To get around this, I would normally sink a single gang flush mounting socket box in the wall behind the programmer to house some additional new terminals used to replace them. In your photo such a box appears to be existing already (lucky you
). As it's metal socket box, make sure that it has an earth connection.
This extra wiring whilst not "rocket science" does mean that you need to be competent with electrical installation procedures to do this safely. If you feel unsure, please get someone professional to do it.
If you are competent to proceed, you will need 4 new extra terminals to replace A, B, C & D. All of the wires presently contained in 'A' go into one of them, all of the wires from 'B' go into the second, 'C' the third, and 'D' the fourth one. Make sure all of the connections are good, and that there is no copper exposed from the terminals that could inadvertently come into contact with the metal back box.
N and L are straightforward they are the same for the Potterton and the Hive, so they simply transfer.
Earths from the EP2002 go to the Hive's earth tether. As there are a lot, you may have to put them into a separate terminal first if required, but they all do need connecting together.
Then it is a matter of moving the remaining wires from the main switching terminals as defined on the back of your EP2002 (1, 3 & 4) as below
to the corresponding terminals at the Hive. Which is straightforward in this instance. [for example 'HW On' (EP2002) corresponds to 'Hot Water On' (Hive)] Even the numbers match up in this case. (lucky you again!
)
The link between L and 5 on the EP2002 does not need to be retained, this connection is made internally by the Hive.
Electrically speaking, as the Hive Dual Channel Version, has the same switching functions as the EP2002, then the operation of the Hive will replicate what the EP2002 does for you now and the way that it interfaces with the existing Vaillant controls.
The only other issue would be if you have an existing room thermostat somewhere. Because the Hive carries out the functions of the room thermostat as well as the EP2002 programmer, 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 three ways, in order of preference, they are.
1. Find the origin of the thermostat cable and disconnect it. Then link the terminals where the two switching wires came from together.
2. Remove the existing thermostat and replace it with a junction box, connect the two switching wires together and isolate the neutral (if there is one)
3. Leave the existing room thermostat in place and put both of the switching wires in 'L' terminal (or set it to the highest setting and hope that no one moves it)
If you need any further assistance with this, post back details of your existing thermostat make / model and what wires are connected to which terminals.