Hi...
Never fished myself but just a thought, try getting intouch with wye and Usk foundation, you will be able to google for them I am sure they would help you out with a tackle shop in the local area or may even know themselves of some usefull info or try gary evans tackle shop the staff in there are always helpfull.
good luck
P.S. I dont know what the natural life is like on Usk but on the peaty acidic resservoirs I sometime fish the naturals seem to be mostly sedges...
Alot of the food will be terrestrial so all the usuall, fly beetle immitations are worth a go such as , black and peacock, alder fly, bibio, small black hoppers, sedges, clan chief and that sort of stuff....