Swallow-tailed Moth.
(Ourapteryx sambucaria)

Resting on a large green lilac leaf - gives you an idea of the size ... about 2" to 2½" from wing-tip to wing-tip.

Information:
Fore-wing length: 22-30mm. At rest, and especially on a dark background, you can see the characteristic pose with wings outstretched and the tail slightly raised in a down-pointing curve. The tail has two dark spots close to each tail-point. The background colour starts off as a pale lemon yellow but fades to white over time. The veins are slightly paler than the background and there are two cross lines on the fore-wing and a single cross line on the hindwing - giving the impression of a stylized capital 'V'.
They are very common in most parts of Britain and fly June-August over most leafy habitats ranging from ivy clad urban walls to gardens and woodlands.