The poplar hawk moth (Laothoe populi) is very different to in appearance to other moths (apart from the eyed hawk-moth which is very similar). The shape and wing formation is unique and the poplar hawk-moth cannot really be mistaken for anything else. It is widespread across the British isles and is quite common.
It is 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 although in good years there can be a second brood in September here in the south. The food plants for the larvae are poplar, aspen, sallow and willow and it is the latter two of these that are common and would usually be the host plant in Dorset. The insect overwinters as a pupa.