how do you access the brake light bulb on a citroen c3?

The hand book says almost all you need to know. There is a 10mm nut to undo at the top of the light cluster - mine was under a plastic dust cover, but other models don't have a dust cover. There is also another nut inside the car, either behind a small plastic cover on the passenger side, or in a recess in the pocket on the drivers side. So far all in the handbook.
I've just spent 2 hours trying to find the final piece of the puzzle, and it is simply that everything is a bit stiff so you need to pull the cluster off and it might be very stiff! There are some plastic lugs on the cluster which fit quite tightly into bodywork holes - just pull hard!
After that it is quite straightforward - disconnect the cable (push in the clip to release the connector) and then release all the plastic clips which holds the lamp holder onto the lens.
Reassemble in reverse order

Man who just lost all his fingernails
October 2011