""As I understand it, Silbury is built on a spur of chalk sticking out in the fertile flood plain" You were paying close attention weren't you !"

Nah, I've known that for ages, in fact I imbibed it with my mother's milk. Together with the fact that bleedin' dead flying bleedin' ants don't rot in the turf so don't indicate a time of year when the turf was cut. ;)