Mark wrote:
Does the dribble only happen at night?
Yes.

Then more likely the pressure reducing valve is letting by, rather than
a duff pressure relief valve. Mains water pressure generally increases
at night & there's no draw off, which would remove any excess pressure
that's crept through.

Does the installation come with a pressure gauge?
No.
You'll probably find there's a blanked-off tapping on the pressure
reducing valve/ multi-function valve where you could fit one. Maybe the
gauge came with the kit and it was left in the back of a plumber's van.
A gauge helps diagnosing problems like this.


Did things improve when the air-bubble was renewed?
Yes, but it didn't fix the problem.

Unlikely to be an air bubble problem. You should refill the cylinder
SLOWLY when you replenish the air bubble. Try recharging it again (air
is free, at present) and then maybe a new pressure reducing valve.