Working through the useful (if imperfect - it's not an easy key to write) Unwin (1981), the fly below, which is largely yellow-brown, bristly and about 3mm long (excluding legs & wings) turns out to be a member of the Lonchopteridae. The antennae consist of short broad segments (the 3rd one almost spherical) with a long terminal bristle (arista), and the wings are somewhat elongate and pointed, the veins running largely longitudinally and also being more or less equally thick and evenly separated.
|Specimen of Lonchoptera|
|Thorax and scutellum of Lonchoptera. The thorax has two bristles (the bases shown by red arrows) while the wings only have cross-veins near the base (e.g. as inidicated by the black arrow)|
|Head of Lonchoptera lutea showing dark antennae (the near-spherical 3rd segment bearing a long arista) and the triangle of orange simple eyes (ocelli) on the top of the head.|
Smith, K.G.V. (1969). Diptera: Lonchopteridae. Handbooks for the Identification of British Insects 10(2ai): 1-9.
Unwin, D.M. (1981). A key to the families of British Diptera. Field Studies 5: 513-553. Available for download here.