I eventually found a master tree up a broad ride going NW from the Forest Lodge area, up toward Beggars Hall Coppice. the master tree had the key signs, an oak tree taller than the surrounding trees, and two likely-looking people stood looking up into it. Before long we saw a female come down towards eye level, and as our eye got in we saw two more butterflies flying at the top of the tree, presumably males.
I did wonder whether the female was a White Admiral, but fortunately someone else there knew what they were doing, and saw the key eye-spot underneath. Here's some dodgy photos, much cropped and expanded.
some more butterflies
|Silver washed Fritillary|
|Master tree straight ahead.|