I've tried all combinations of holding the switch down for x number of seconds.
I've found numerous manuals which show a wire shaped tool being stuck into a hole in the carpet in the front-left seat foot-well.
I can't find any such hole in my 2008 Picasso Grand's front left or right seat foot-well!

I do have the tool though. Please could someone post a picture to help me find where this tool needs to go to release my handbrake! I've just spent 2 hours examining the front foot-wells from all angles, so any pictures from anyone who knows what they're looking for would make me very happy!
