Poplar Hawkmoth (Laothoe populi)
It is also a big moth and if you see it fluttering around a light you could easily think at first that it was a bat. It must rank as one of Britain's largest insects I would have thought.
It does vary in colour between this almost blue to a much lighter shade of brown. There is also a buff version found, notably, in the London area. These browner versions tend to be the females.
This is a moth readily attracted to light and is single brooded flying from May until July. In good years there can be a second brood in September. The food plant for the larvae is on Poplar, Aspen, Sallow and Willow and it is the latter two that are quite common and would most usually be the host plant in Dorset. The insect overwinters as a pupa.