I came across Blue Dun Flies via Simon Graham. This mayfly pattern is interesting. I am not sure if the proposed material: Aero Dry Wing, a poly yarn material, is readily available or any different than other poly yarn materials. The comments here and there re the material state it is hollow and straight making it appealing for flotation and handling.
Dave Wiltshire at the Fly Forums (UK) offers a source for this ‘expensive’ material at Flytek (UK). Perhaps a visitor can comment re this material’s qualities and availabilities? I believe Hareline & Wapsi may also provide this material.