When I spotted this moth on a leaf it stood out because of its interesting wing shape. The Herald moth - Scoliopteryx libatrix.
It apparently flies in Spring and Autumn - intending to over-winter as an adult. That doesn't bode well for our weather as its only July, but maybe they're going to have another cycle of breeding this year.
The larvae eat tree leaves (willow and poplar), so no threat to the allotment. I'd happily see this interesting visitor again!