Four-lined Horsefly (Atylotus rusticus)
This horse-fly is now very rare in the UK, with its status is recorded as such on the RDB1 (Red Data Book Species).
A. rusticus is mostly found on southern coastal grazing marsh, especially within East Sussex, which is the type of environment in which I found this specimen, albeit a little distance away from where most existing records show it to be.
The only other known site of significance is many miles north and inland, close to Oxford and in the Cambridgeshire Fens. The larvae are probably predatory in wet plant debris of freshwater ditches and pools which characterise both locations.
Alas no video for this one as it flew away before I could film it.
However, given its rarity, I feel very fortunate to have managed a couple of photographs at least.