I parked the car in a car park, on a slight slope, and when I returned 10 hours later, it had rolled across one traffic lane, and into a new parking bay.
The dealer said there was nothing wrong with the handbrake. Yes, I know I should have left it in gear, and now I do, but I still feel the handbrake should have held it. Luckily no damage was done.
Whilst I was in the dealer's reception area, another customer heard me explaining the problem to the service manager, came over and said she had brought her Jazz in for a handbrake check because it had rolled down her drive-in and hit a visitor's car parked in front of hers. Taken with other comments on this site, there seems to be a pattern.
Funny, because I have seen a Nissan role out into the street in front of my house, a Audi that moved itself because it was not in gear. It is not smart to leave a car in neutrel with the brake on. there is nothing holding the weight of the car down, but the rear brakes. at least with it in gear it will be locked better, but you still need to have the brake on.