Install Nest over Potterton EP2002 Y-Plan

So there I was I planned everything in advanced for my father's home. He sent me all the details of our current boiler and heating system and I was convinced that it was a y plan system because i have a 3 port valve that says "y plan". I arrive to do the job and I see this silly wire set up. Why can't they just set it up normally. So now I'm wondering how to decode this and wire it up to the nest 3rd gen properly. N and L are simple enough. What's the business going on in terminal B and D. I could do with some help please.

My setup is as follows:

Potterton Suprima Boiler
Potterton Ep2002 programmer
Potterton prt 2 room thermostat
Potterton ptt cylinder thermostat
Honeywell sundial y plan 3 position diverter valve
Gundfoss pump

I haven't started the job just yet I want to make sure I've got the wiring right way round in my head.

 

A-D are spare terminals used for linking wires as required,they do not take part in any switching.

Is the existing programmer set to gravity operation ?

 

Hello, thanks for the response. It appears to be in the fully pumped position as the little switch is in "-" position.

 

So I plan on doing the following:

Potterton -> Nest
N - N
L-L
3-6
4-3

In a separate block:
Connect all B together
Connect all D together

Then on the heatlink:
L-2-5 (using the wire left from L-5 from old programmer)

Then on the room thermostat
H-TL
Terminate neutral

 

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