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You need a permanent live feed to the nest hub. Then a feed goes to the hot water and one to heating.
You only need the existing thermostat cable if you want to mount the display unit where the current thermostat is.
This site gives a good explanation for Y-plan heating systems.
Mr_Sworld
@Backstop
Anyone know of some slightly more advanced wiring diagram for a NEST installation? My Valiant boiler has a single 230V RT terminal and a 12V twin terminal. The NEST supplied diagrams are pretty vague and suggest routing the live conductor via the heat link to be switched by the thermostat.