Thank you all for your comments.
Ann, the first was shot with the Fuji 18-55 zoom, usually sold as a kit lens with the camera. It performed remarkably well on the day. It is quite a dangerous sport, which increases the thrill level.
Bent, it is a great sport to practice panning, as the riders are constantly zipping past so there are lots of opportunities to try it out.
Akira, this event was south of London, so rather a long way from Robert's event.
Tommie, this was my first time at motocross. It was a lot of fun, but very noisy. I would use ear protectors if I was a regular.