Taken from Edmonds & Lee - here's the dressing for "No. 2 Waterhen Bloa":

Hook 1 or 2.
WINGS.- Hackled with a smokey grey feather from the under coverts of a Waterhen's wing. (The darker side of the feather towards the head of the fly).
BODY.- Yellow silk No. 4, dubbed with Mole's fur.
HEAD.- Yellow silk.

March to end of April.