Julian Cope presents Head Heritage

I've had some success contacting these guys re damage to sites:
http://www.english-heritage.org.uk/about/contact-us/offices/southwest

Otherwise presumably its a scheduled monument so the farmer is breaking the law by failing to protect it. Ring 101 and report it as criminal damage. Ask to be transferred to Wiltshire Police.

Let us know if you get anywhere, if not I'll pass it on to the EH bod in charge of ARCH.

I've seen similar old damage at a few sites, it's unbelievable what some people allow to happen.