Gamlingay Wood, Gamlingay, Cambridgeshire.
Free public access, with free parking along the side of the access track.
An ancient wood on heavy soil, many of the paths are mostly too narrow to encourage nectar plants and butterflies. The central path leading to Sugley Wood was clear enough in 2012 for brambles and thistles to flower - to the benefit of the Silver washed Fritillaries.

Personal sightlings:

22 July 2012. Central east-west ride near the exit to Sugley Wood. 2 Silver-washed Fritillaries, and a few Ringlets.
26 July 2012. Same place, 3 Silver-washed Fritillaries.
30 June 2013. 1 Small Tortoiseshell, 4 Speckled Woods, 2 Meadow Browns.