Look inside the boot behind the lights and there's a small plastic disc set into the trim. Remove this to get to the bolt securing the cluster. Undo the nut using the screwdriver in the kit in the spare wheel recess.
Now you need to ease off the cluster from the outside which is where it gets tricky. The back of the cluster (edge near the tailgate) will move quite freely but there are two metal lugs on the front edge of the cluster and it can be a right pain to get these to pop out of their holes. You need to rock it back and forth easing the cluster out a little at a time.
Once the cluster is out there's a plastic moulding on the back it which contains all the bulbs, you need to release the four clips and remove the moulding to get at the bulbs.
One thing I've never worked out is how you're supposed to get the plastic disc back into the trim after you're done. It looks totally impossible to me which is why I now have two holes in the trim where the disc was