Issue with Nest not firing up Boiler

I am having an issue with my heating system and wondering if anyone might know what the issue is.

For background I have a simple two zone system. One for heating, the other for hot water. Both are controlled by a Nest thermostat. Up to today all was working fine.

Today, when I adjust the Nest for heating the boiler does not fire up. I can see the motorised valve gets activated and is opening fine but no signal to the boiler. If I control the water from the Nest it does fire up the boiler. Also if I select the manual override on the Nest’s heat link it also fires up the boiler and heats the rads. I’m at a loss as to what is preventing the Nest from firing the boiler without using the manual override. When trying to turn it on from the Nest it turns orange as normal and says heating.

Anyone any idea? I suspect it’s going to need a call to a plumber but hoping somebody may have experienced similar before and may know what the issue is.

Thanks in advance.