Thanks for reaching out to us @Oskeates and for all the details. I'll do my best to try and assist you in this situation.
The activation from the Heat Link is called the manual mode, this is an override that activates the heating and hot water, if available, independent of the display.
Let us check a few details together first to get a better idea of what's going on:
1. Is this a new installation or has it been installed and working for some time before this behaviour occurred?
2. Was it installed by a Nest Pro or professional installer?
3. When you put it in manual mode from the Heat Link do you get any message on the thermostat display about the manual mode?
4. Do you only control the heating from the thermostat or do you also have hot water control?
Looking forward to your reply!
