Seriously, I've found that the Grey Wulff is far and away the best pattern for Derg. Last fished the mayfly there in '07 and the Wulff was the first fly out of the box on Day One, I proceeded to catch on it and never felt the need to take it off again. My boat partner faffed around with various patterns until he, too, recognised what was happening and stuck on the Grey Wulff. Other boats reported success to the pattern as well.
We did have some success on Walker's Spent Gnat in and around Luska Bay and behind Illaunmore in the evenings. Hope you have a great week