...well, for one thing Nige', rabbits haven't been 'at it' (burrowing at Silbury that is) for millennia; in fact they haven't even been at it at Silbury for even a millenium. Rabbits were introduced to Britain from France in the 12th century. God knows how long it took the little buggers to get to Silbury but even if they made it during the 12th century that would mean the've only been there for around 900 years - not 2000. Come to think of it though, that does equate to a friggin' lot of rabbits.