It's quite simple once you know how. It's not obvious though.
Open the tailgate. On the inboard side of each tail light you'll see a circular plastic insert about half way up the side of the light. This insert is part of the boot trim plastic. Prise off this insert with a flat bladed screwdriver. Inside you will see a bolt head. Can't remember if it's allen or hex. Anyway loosen this (no need to remove). Once loose the whole light unit will come away from the car (if you pull on it gently). The edge nearest the front of the car (round the side) is held on by two plastic tabs that insert into rubber grommets. The light should pivot here and come away from the grommets.