Does anyody know how to get access to the handbrake cable adjustment nut under the armrest on an Octavia 1.9tdi Elegance?

Yep

Pull the trim off the vertical part of the armrest (It's in two sections which need to be pulled off to the sides, so you'll have to put both front seats forward), under it you will see a bolt which holds the top of the armrest to it's base. Remove this nut and put the armrest to one side. Next, remove the rear ashtray/cup holder by pulling it directly up (try not to pull it back or you risk damaging it's mounting tags. Once the cup holder is out, you will see two torx screws (about T20-T25 IIRC), remove these and then you will be able to lift the back of the central console. As you lift, be ready to unplug the wiring to the cigarette lighter, once you've done that, withdraw the plastic completely (taking care not to damage the handbrake lever on the way. You will see the handbrake adjustment nut.

CheeRS VRSPhil.