Only poverty spec 206's had a keyhole for a manual boot lock.
Normally its one of the door lock motors that goes wrong & causes the central locking to play up, later ones have been modified by Peugeot.
There is an emergency way op locking / unlocking the hatch.
Open the hatch, that big bit of plastic trim on the inside is held on with metal clips, grab the bulge bit & pull hard to release the clips ( it sounds brutal ) & remove the trim.
You will then see the locking solenoid, just push it over to the lock position & then shut the hatch again to lock it, once its locked this way you wont be able to open it from outside, you will have to fold the rear seats down & reach in through the passenger cabin.
When you come to put that hatch inner trim back on theres a plastic peg on it, that lines up with a slot in the tailgate, line it all up & smack it back on to secure the clips.
If you cant ever open the hatch its fold the rear seats down & go in to the boot that way.
Lee, June 2007