We used to hold a 24-hour marathon at the Young Chelsea every 18 months, with big prize money and players from all over Europe. I played in it five times and directed it three times. The standard format was 56 pairs in an all-play all movement of 165 boards, half-hour breaks between sessions. It was very popular and over-subscribed, but gradually interest waned (perhaps as everyone got older, and 24 hour bridge on the internet became available), until the last one (which was won by gnasher and his partner) only attracted 18 pairs, and we've decided to wait until there seems to be sufficient interest before holding another one. Hopefully your event will be able to re-awaken some of that interest and attract some of the players who used to enjoy the YC one. In particular there was a group of Swedes who regularly flew over to London for the weekend. Good luck!