it is pretty straight forward, I have see one being changed. You just need to remove the the trim pushing it towards the window inside the open tail door, then use small flat screwdriver to release two plastic clips one each side it is not evident and clips break easily.
Thanks - in the end I chickened out and took it to the local dealer, and watched them struggle with trim, fiddly clips and taking the bulb housing off from the outside too. Guess it's easier when it's not your car - you're probably less wary of snapping a piece of plastic trim. What's wrong with a couple of screws holding the part in place? Anway, fair play to the garage; their service manager came out, swapped the bulb for me and only charged me the £1.60 for the bulb.