All interior lights and puddle (lower door) lights not working
 

This is vexing me. I have a 2001 VW Sharan 1.9TDi. None of my interior (courtesy) lights work and nor do my door puddle lights. I have checked the fuses, Nos 27 and 28, and on inspection and because the number plate lights and remote locking are working I know the fuses are OK.Because none of my interior lights work I can only assume there is a problem with the power supply dedicated to the interior lights.
Can anybody tell me what (and where) I should look for? Is there an in-line fuse I am not aware of, some kind of relay, or a control unit? Are the interior lights controlled by the central locking control unit or is there a seperate one for the lights?