Thought this posting to uk-leps yahoo group may be of interest:-
We drove through heavy rain to get to Whipsnade for lunch in a pub. Then went for a walk along the tops of the downs starting at the Chiltern Gateway visitor centre. By now a sunny gap between the showers had moved in. Lots of meadow browns, a few Marbled Whites and Burnets, the odd Gatekeeper seen. But the highlight was back at the visitor centre where a large odd butterfly was being blown along in the wind. It stopped on the grass and was almost trodden on by kids. It shot off downwind. I went out and took a look, and found it on the glass wall of the visitor centre. Purple Emperor was the last butterfly I expected to see up there! And in the middle of the day too. With its blue irridescence and markings it was unmistakable. I rescued it and took it to a quieter spot away from the kids dogs and parents. After resting a fair while on my hand it started to walk up my arm and then launched and disappeared downwind and was gone. It seemed fairly weak so I guess it was getting elderly. However it looked in good condition. I've only ever once seen a purple emperor from a distance a long time ago. so a very pleasant surprise to find one in an unexpected location today. I didn't know there were any at all in the Chilterns?
I managed to upload a couple of pics here: