Riding trails and circuits on a mountain bike can be immense fun. It is likely to be fairly muddy and the favoured garb is rather more rugged than the pristine lycra chosen by road cyclists. The peak season for mountain biking is winter and spring. A decent bike might be £600-£700, but the full on top spec race machine could run to £3,000 or more.
Mountain Bike racing is controlled by British Cycling whose site includes details of upcoming events nationwide. There are 5 or 6 national events which are based on an hour of cross country riding on a circuit. Most activity, though, is regional and does not require much of the formality of licences and categories attached to road racing. There are MB “Meccas” known as Trail Centres in Scotland, Wales, the Lake District, Kielder and so on. Possibly the most convenient centres for us are Llandegla and Cannock Chase
Various event styles exist and there are short and long events in the cross-country category(even 12 hours or 24 hours, which require serious endurance) . There are also downhill races which are for the near-suicidal being a bit like downhill skiing but with hard obstacles such as rocks and trees to negotiate.