Rabbits are mad. They have a mania for constant burrowing. At any one time, a good percentage of the surface of the hill is spoil, brought up from several feet below the surface. The base of the hill has expanded beyond the fence due to rabbits.

You're right, they've been at it for millenia and the whole surface has been churned many times. But it's a question of numbers, and no control. I bet there are more there now than at any time. Silbury is the only place in Wiltshire where rabbits can get in and foxes can't.

It's not just the damage, it's the blatant crass failure to act as guardians of something so important that drives me wild. That, and the sodding, leering prat of a jester that's to be the public face of EH next season.
:)
Can't cut the scrub because they don't have the insurance? Well they have £5,000,000 cover for members of the public who might get hurt on their properties, so it ain't rocket science.