It's always been a popular event, and seems to be getting more so. They have a good event formula, which they stick to and seems to appeal to many.
The venue changes (in Shropshire) each year and the exact course is kept secret till you arrive at the hall. Then you pick up the route and mark it on your map etc. before the start. This means that navigation comes into it as nobody has reccied or knows the route from before, almost basic orienteering. Add food, drink, good countryside, winter weatherand it is fun for everyone.