C4 Picasso Key Fob Reset

Hi There,
I'm sure this must have been answered but no amount of searching pitched-up an answer so...
A 2007 2.0 diesel Picasso
Battery replaced in the key fob and...

(wait for it

OH, you know the punchline. The remote lock & unlock doesn't work.
What did i miss? The battery is new and a multi-meter confirms the expected voltage.

Appreciate your thoughts on how to reset the fob.
Cheers,
Marti