Hi @Suts . I would be happy to try and assist you.
You mentioned all the breakers and switches look good, that is great.
If you have not already try and power cycle the Heat Link, if you're comfortable in doing so. Turn the power to the Heat Link off for a few minutes and then turn it back on. See if it has any reaction after when you press the button on it.
Thanks for your response @AlexD . In the end I got I touch with Google support who issued me a new device as it was still under warranty. You are required to send the old heat link back and you can also claim up to £100 back if you need to get a professional to install the new one. I just took a picture of the wiring before I took off the old one and rewired the new one. I had to reset the thermostat but overall it wasn't too difficult.