Hi Alistair,
i'm far from an expert (you have to watch what you say these days

) but looking at the pic i think it's a pattern swimming with the hook-point up, as the thorax is made of two large goldhead beads with the wingcases mounted underneath. The abdomen looks woven, making it overall a pretty difficult pattern for a beginning tier, as Mark stated. You can simplify things by making a thick abdomen in black dubbing, ribbed with a 4 lb nylon. Put some lead wire under the abdomen and thorax (also black dubbing) and cover the latest with a black dyed pheasant tail wingcase (you can keep the nylon to make a segmented one), tails and (optional) legs are black goose biots. This insect will also fish on its back (as most leaded larvae and nymphs do) which is no problem as the stonefly larvae or plecoptera do not swim, but creep. When taken by the current they retract their legs, therefore i often omit them in my patterns (combing out the dubbing in the thorax will enhance the impression of movement). Sorry i can't send you any pictures. Hope this helps.
Kind regards,
Johan