Nest heat link boiler wiring help

[paweas]

Hi i am having problems with wiring my nest heatlink into my boiler need some help please.....my old thermostat has been removed so i need to connect this at the boiler end i have two 3 core cables going into my boiler one to the thermostat and one into the pump but i'm not sure which wire to connect to the nest heat link........i'm guessing the thick grey cable is the main power to the boiler and the white cable was from the programmer......thinks its best just to wire it in at the boiler house as the cables inside are confusing and not labeled

 

[paweas]

 

[paweas]

i connected the live from the white thermostat cable and the boiler powers on but i cannot shut it off via the heatlink switch its on all the time

 

[paweas]

Should i be using the live from the pump instead of the thermostat to call for heat on ? or doese the signal come from the thermostat ?