Good job Sophie....bags of energy in that first one (that and the rider is on a bright blue Orange....bikes don't get any better than that )
Looking at the set on Flickr pretty much all of the shots look like the rider is on man made surfaces rather than negotiating obstacles on a trail. Was that bridge thing a temporary river crossing? (I hate when they put them in....pelting into a river at full speed is great fun).
For cross country racing I think there's a couple of main things to try and capture....the fitness side of thing with the riders grinding it out along a wide, easy section and the technical side of things where the riders have deal with obstacles like rocks, roots, loose surfaces, dodging trees, corners, jumps, etc, etc.....