Nest 3rd gen + programmer advice

[Philz55]

Looking to get a pro to install my Nest 3rd gen but I like to get a general understanding of the install. System is WB boiler on S plan and HW storage cylinder
I would like to use existing wall stat wiring to power the 12v thermostat from Heatlink T1 T2 in tank cupboard. I've got a reasonable understanding of the wiring configuration but not sure what happens with the programmer located with the boiler.
Thanks.

 

[muggles]

Programmer in the boiler will need to be bypassed or, if that's not possible, set to be permanently on

 

[ianmcd]

if you use T1 &T2 to power the nest you also need to use the earth wire