One of the first group of insects to appear in spring are the Bee-flies. They are usuall seen on warm sunny mornings taking nectar from a huge variety of flowers. Later in the summer you might be lucky enought to see them hovering at the entrance to a wasp nest, waiting for the chance to flick their eggs into the nest where their larvae will feed on detritus left by the wasps.
The species you are most likely to see is the Dark-edged Bee-fly.
Last year I noticed a smaller type, feeding on Wild Carrot on a sunny bank in Wareham Forest. This was a rare Heath Bee-fly, the first and still the only one of this species I have ever found.