I know the feeling too. I managed to get a willing OT a few days ago myself, but the day before I was chasing holly blue at Brown's Folly and spotted a female with wings half open right in front of me. Then I dropped a lens cap, bent down to pick it up and off flew the butterfly. The lens cap cost 99p, and but for the litter, I would have gladly left it there to have got that shot.
![Sad :(](./images/smilies/icon_sad.gif)
As for the purple emperor at Bentley, I doubt if I'll make the trek as it's a fair old way from my neck of the woods.