Thanks for the replies! I'm hoping that seven hours was an anomaly, due to a) us having to have a 24 hour break in the middle (so the game was spread over 2 days) and b) us all learning what the hell we were meant to be doing. It bodes well that you can get the game in quicker though. Once we worked out what was going on everything kind of clicked, but there was much rifling of pages and gnashing of teeth beforehand.
I find it kinda funny that a £100 game is a poor man's game! I'd love to play KDM but I don't want to take a loan out for a single game. How good is it, if you don't mind me asking?