You need to get a small mirror and hold it between the bulkhead and the metal box. You will see a slot (hole) with the edge of a small wheel showing. This edge of the wheel has holes all around it. Get a small allen key or other similar tool and use the holes in the wheel to turn it. It needs to be turned clockwise, so this is an upward movement (you'll understand when you see it!). This EVENTUALLY will release the metal cover.
Once you get the wheel turning it is fairly easy to do, but it takes a lot of turns to undo. Hope this helps anyone with the same query. I had to ring my local Renault dealer to find it out. This is exactly what they told me they do. Bad design if you ask me!
Matt, October 2007
