There are 2 parts to the UK system.

I have a hot water tank in my system, an there is a junction box there, one part of the Nest system goes there this is what they call the "Heat Link". It takes it's power from the 240v feed there and uses the cable that ran to the old thermostat to provide 12v to the nest wall unit.

So all I can see is my nice nest thermostat on the wall, no ugly cables just a straight swap for the old thermostat.

Take today for example the nest was saying it was 20 degrees this morning in the house, and it now says it's 21. But the swing I notice in the house is much greater than that.

The heating hasn't been on all day as I've set it to only heat to 18. So of course it is sat there waiting for it to get cooler than that.

I need to go buy another digital thermometer I think!

Cheers

Cris