Confidence: I'm as sure as I can be.
Notes: The lack of patterning, three tails and the reddish colour mean that this is a mayfly of either the Leptophlebiidae or Ephemerellidae family. If you zoom in, you can see very small detatched veins on the outer edges of the forewings, in between the main veins. This makes it Ephemerellidae. Of the two species in this family, Serratella ignita is by far the most common, although there is a small possibility that this could be Ephemerella notata.