Confidence: I'm as sure as I can be.
Notes: Either Phryganea grandis or Phryganea bipunctata. Both are very large caddis flies, the largest in Britain. I'm not sure the exact species can be determined from this image. Seperating the 2 species from images alone is difficult, size differences are minimal- a female P. grandis can have a forewing length of 28mm and a female bipunctata 26mm.