I had a couple of Privet Hawk moths in my garden a couple of weeks ago and they are
seriously large insects. Very dark and with a wingspan just about as large as you are likely to find anywhere in the UK... 11cm+. If you see one flying it would be
very fast and generally, during the day, this will be only when disturbed. A short flight looking for somewhere else to hide.
Beautiful moth and most often seen sitting on their usual position of a wooden post or shed during the day. After dark, they will fly more, but obviously not often seen.
This was the one in my garden.
Trev