According to M.Joshi :

We have recently had a new Vaillant ecoTec boiler and central heating
system fitted.

Whenever the system is on, a stream of water drips from the flue
outside occasionally?

What is likely to be causing this and is it easy to sort out?

Probably condensation of water vapor in the exhaust. This is probably
entirely normal - especially so if the unit is high efficiency
and the flue is plastic pipe.

Call the installer and ask.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.