I reckon this weeks photo assignment should be waiting for these tools and getting a picture of their plates both on the bikes as well as their car/van etc lower down the hill. A picture of their mugs wouldn’t go a miss either.
Simon I would suggest contacting the local Police and offing them your photo. It could help in “building” a case which may require photo evidence to support it.
Riding an MX bike badly and without consideration to others an the environment is stupid to say the least but throwing rocks is downright dangerous and completely unacceptable in my book they need reporting asap.