This is the way.
1, In the left lower boot are some push in trim clips holding the lower carpet type material. Remove these clips in the region of the rear light unit.
2, Pull the trim down and look under the plastic parcel shelf support in the back corner of the car.
3, Look in the small square aperture (hole?) that was hidden by the carpet trim that you pulled down!!
4, In this hole a black plastic type wing nut will be seen screwed on what looks like a piece of threaded rod. A torch might be useful to see. Remove the wing nut. This is awkward as the access is not quite big enough for an average size hand!!
I needed some pin nose pliers to get it started.5, Grasp the light unit firmly and pull at the bottom. Ours was very tight but did move with a jiggle. Once the bottom is out the top will unclip.
6, Remove the lamp holder from the lamp unit and change the bulb.
7, Refitting is a reversal of the above but make sure the top of the lamp unit is located properly or else it won't sit flush.
It took us 2 hours because the book is useless!
and we stripped the back of the car into little pieces!I reckon 1/2 hour now would see it done.
Good luck.