at this time of the year look for rises. these are small splashes or dimplles on the surface of the river caused by trout and grayling coming up to intercept a fly wich has got caught in the surface film. if your not sure what fly they are taking then try an 'f' fly or an adams. these are general flys which cover loads of differnet flys. if you have caught one of the flys they are rising to then find a fly in your box wich matches that fly the most. if there not rising then try one or two of the following nymphs:
goldhead hares ear, sawyers killer bug, pink shrimp, partridge and orange, pheas