I'm not much of a Dipterist but there is some interesting information, and a key to the genus, in Verrall's British Flies, on p. 328. Fortunately this rather rare volume is now out of copyright and available in the internet for free download as a PDF. http://www.biodiversitylibrary.org/item/36929#10. While you are there you can get Fowler's Coleoptera and Morleys' Ichneumonologica and be all set for a wild night in.
it is probably H. pulvialis. This is a common fly and is browner than the H. crassicornis. There is an excellent book called British Soldierflies and their Allies by Stubbs and Drake which deals with this family of flies.
I downloaded Verrall's book which has highly detailed descriptions of these species, both sexes. From this I am sure it is pluvialis. A key feature is the presence of a rusty colour to part of the antenna as opposed to completely black. This does not show up too well in some photographs because of lighting, but tweaking the levels in a photo editor should reveal it (or not). All of my photos of clegs are pluvialis, including a male. They are all from the same locality. Incidentally, Verrall describes his personal progression through the week of the aftermath of a cleg bite. In general I react similarly. They are sods!