I sent the photo for confirmation to Robert Homan, the County Moth Recorder for V-c33, East Gloucestershire, who kindly supplied some supplementary information.
The Herald overwinters as an adult, and Robert suggest it may have been drawn out of hibernation by the recent brief outbreak of spring. I asked if it was called the Herald because it heralds the spring. He dashed such notions, saying that in “The Scientific Names of the British Lepidoptera – their history and meaning” A Maitland Emmet suggests that the “herald” might be derived from Linnaeus’ original name of libatrix – “the moths’ vestiture may have suggested to him a stately robe, suitable for ceremonial duty.
Robert has provided this distribution map of the Herald in East Gloucestershire which shows a close parallel with the main river systems and thus its larval food plants – willows and poplars – both of which occur in my garden.