Amazing photo by Tony Rogers of a female feeding on oak trunk close to a Purple Hairstreak, from Knepp -
Besides Emperors, these tiny sap bleeds (if that's what they are) on stressed or diseased oaks are visited regularly by Red Admirals and Purple Hairstreaks, and also by the occasional Comma and Speckled Wood. White Admiral (scarce at Knepp) will also visit.
Purple Hairstreak has been in great numbers this season, at least until last Tuesday's rain, and has been visiting bramble flowers frequently in the hot weather. I hadn't seen it on flowers since at least 2006, the last decent summer we had -