Nest 3rd generation UK Install Terminals 4, 5 and 6 are used to connect cables that switch your domestic hot water on or off
From the following documents it seems to me that 2 &amp; 3 (common and call-for-heat) are used to provide power to the zone valve (which presumably then provides power to pump and boiler as in a conventional system)

This assumes you installed the Nest-display so that it repurposed the old 240V thermostat wiring for 12V power from the Nest Heat-Link unit (T1,T2).

You should check this with the installation manual supplied by Nest or using the online configuration tools on the Nest web-site.


Nest Thermostat Installation Guide
How to install your Nest Learning Thermostat