D.M. Procida wrote:
We have a cold house this morning...

The Flame LED blinks, and so does the Fan LED (which indicates a "flame
or flue fault").

We've called a plumber. In the meantime, what's the most likely fault?

Daniele

Don't always expect pinpoint accuracy from these fault lights.

In this weather I'd suspect a frozen condensate drain if you have an
external one as a prime suspect.
Fan motor failure can occur anytime but not common in my (limited)
experience.
If it has not had comprehensive servicing for a long time, the bottom of
the heat exchanger/sump could be full of crap.