Probably being a bit of an idiot here but how do you change the bulb in the boot? Pulled the plastic cover out but then there's another black box like thing covering the bulb.

Admittedly I did only look for 5 seconds before giving up. Changed all the other bulbs without hassle.

Easy mate. Detach the 'black box' small screw driver to up clip it so you can access the bulb, fiddly but doable. Carefully shimy out the bulb and replace.