I've been tying a few dry flies that I've found to be exceptionally good for grayling, but also wild brown trout. I don't know if this fly has a name, I call it the NG (Nifty Gnat), apologies if anyone out there has already put a name to this pattern. If you have let me know and I'll credit you.
It's dead easy to tie and and very effective, it's a variant of a Shipman's buzzer and Griffith's gnat all rolled into one:
Hook size: I tie them on sizes 16 down to 20
Thread: Claret with diameter to suit hook size
Breathers: Poly yarn (I've also used foam)
Hackle: Purple (or grizzle) cape top
All materials available from http://www.funkyflytying.co.uk