have you filled the police form out yet?

do that first

odds are that it won't be covered by the insurance because it's not on the public road so you might have to fork out for yourself

alternativly you can get the police to let you know who his insurance company is and then go to them direct, this has been known to work in the past where they circumnavigate your incuarance company so you don't get a blot on your record