Near everything you need is on the internet This Installation and Service Manual Shows page 4 fig 1 how connected in the boiler. For the thermostat seems there are various models This is one model a quick look and it seems these build into boiler so you would need to remove and wiring in replacement independently.
But just like I have just done you can cut and past the details in your post into google and get results.
The question I ask is why do you want Nest? I understand the whole idea of switching heating on before one gets home, but I have eTRV's fitted in mothers house which will follow Nest however one you can change the valves direct anyway, and two the reaction time is slow need to turn it up some hours before return so whole idea rather pointless with slow reaction time.
I would say either go the whole way with automation with something like EvoHome, or don't bother.
The above is based on the boiler terminals from the manual posted by Ericmark. As he also stated, any existing built in controls will need to either be set to be permanently 'on' or decommissioned. If you need any further help with that, post back details of the unit.
You can use a separate plug in power supply to power the thermostat if you don't want to connect it to T1 & T2 at the Heatlink