Favonius quercus - Purple Hairstreak
I have encountered this species on many occasions now. The adults remain high in the canopy, especially favouring large Oak trees, descending rarely to a level where a butterfly photograph can be obtained.
They often go un-noticed as they intermittently spin up in territorial disputes at the top of such trees.
The males and females are quite differently marked on the upperside, the males having a purple sheen only visible from certain angles, whilst the females have a block of colour as shown above.