Move the black box behind the rocker out the way first and its a very easy job mate, dont move the rail and its fookin solid and will piss you right off.
Basically, two bolts on the black box, one each side, and you have to disconnect some clips.
Two bolts on the top of the rocker, careful with these as the threads might shear off.
Watch out for all the fiddly bits underneath the rocker when taking off, you really dont want these to fall into the engine.
Also your gonna need to buy another gasket, the rubber seal around the bottom of the rocker, from citroen or ebay, i got one for about a fiver. The current one will be all crusty and useless when its removed