In my experience a lot of people have trouble with this genus because the traditional features used in keys are quite variable. In particular it is difficult to separate the common Thelaira nigripes from the rare solivaga.
All keys use the size of the outer-vertical bristles and the anterodorsal bristles on the middle tibia, which are a bit variable, and Belshaw (1993) uses the colour of the abdomen, which is one of the worst features to rely on – it is a confirmatory feature at best. This leads to a lot of Thelaira nigripes being wrongly classified as Thelaira solivaga – the 2 species are very similar but they can be distinguished if the correct features are used.
Belshaw is a little ambiguous when he talks of the middle leg only having “2 long bristles on its anterodorsal surface”. What constitutes “long”? All Thelaira have both long and short bristles on the ad surface of the mid tibia but I prefer to ignore the smaller ones at the top and bottom of the tibia and focus on the rest, in the middle. Here are some photos of Thelaira nigripes tibiae:
Here is a photo of a Thelaira solivaga mid tibia:
Also, a very useful approach is to look at the male genitalia – here is a nice set of figures from an article I saw a while ago:
43 – Thelaira leucozona (Panzer, 1809); 44 – Thelaira solivaga (Harris, 1780); 45 – Thelaira nigripes (Fabricius, 1794); 46 – Phenicellia haematodes (Meigen, 1824) (not British).
Pay particular attention to the segment just before the genitalia themselves and the way that the surstylus curves. The distinction between nigripes and solivaga is still very fine but in conjunction with the other features you should be able to make a more confident determination. Later I will add some closeup photos of the male genitalia.