I had two of the bulbs out (one failed - one down to corrosion). As I recall they are about £2 each from Halfords.
I've forgotten what their real name is - capless bulbs? They do not have the metal base and are in white plastic mountings - twist a quarter turn and pull to remove.
You have to remove the lining from the boot lid - about 5 twistlock plastic screws - again, about a quarter turn to remove.
Once the lining is out of the way you can see the row of four bulbs in their plastic housings on the two parallel power rails.
Rotate the failed ones a quarter turn anti-clockwise and they should(!) come out easily.
Remove the old bulb from the plastic housing and replace with the new, then re-insert.
I had a lot of corrosion on the power rail around one of the bulbs which was causing bad contacts - I've forgotten how I cleaned it up, scraped it with a screwdriver probably, but I'm sure there's better ways!
