Hi,
I have a Myson programmer installed - with a Honeywell T40 thermostat (connecting back to a Potterton Suprima 50L ... on my 3rd PCB in 10 years!!!!).
I'm looking to replace the Myson with a Nest 3rd Gen (UK with heatlink).
The Myson has live / neutral - and then the black and grey are...