I wanted to stop by and see how I could help.
Thanks for assisting, @David_K.
@Huxley51, It appears that you might have a dual transformer system with two separate power wires. If this is the case, only the Nest Learning Thermostat would be compatible with your system. It would also work with the Nest Thermostat E or the Nest Thermostat 2020 version, but it needs to be rewired by an HVAC technician or a Nest Pro. Please take a look at this article and let us know if you need more help.