Devil's Dyke, Cambridgeshire. An ancient defensive barrier across the chalk over Newmarket Heath to the edge of the Fens.
It is a public footpath with access at all times from each end and from a car park between Swaffham Prior and Burwell.
I have seen large numbers of Brimstones here in spring as well as Green Hairstreaks and Orange Tips.
The Speckled Wood was by the path through the bushes from Burwell Cutting.
Also strong colonies of Small Heaths and a couple of Marbled Whites were seen on 26 June 2011.
The Devil's Dyke is famous for its Chalkhill Blues flying in July.

Wikipedia entry . Restoration Project