That's an Anthopotamus distinctus female subimago (dun) (Commonly known to anglers as Potamanthus or Golden Drake)
I wish I had been down by the river that day! That looks to have been a nice hatch.
Often I check the lights before going to work and end up with torturous images of mayflies swimming in my head all day long!!
So, it is correct to call them potamanthids but not Potamanthus. Confused a little?
Speaking of latin, potamanthus translates into "river flower".
Where's Roger when we need him? :)
However, I believe (based on eye location on the head) that it's actually a male.
At first I thought so too, but the specimen seems a little light in the special equipment.