How to self install Nest Smart Thermostat v3 (Y-Plan Potterton EP2002)

How to install a Nest V3 Thermostat with hot water control onto an existing Y-Plan System with a Potterton EP2002 programmer. TURN OFF FROM MAINS BEFORE DOING ANY WORK! Explaination of Y plan system Expplaination of Y Plan wiring diagram Removing old thermostat / programmer Best tips. Instructions are similar for all Y-Plan systems with a programmer.