I've been a couple of times to Lathkill Dale, Derbyshire, to photograph the White Throated Dipper, Cinclus cinclus. So called because of their habit of dipping under to catch bugs and other tasty morsels. It's a strenuous walk from the car park in Over Haddon to the river, all downhill, and then you have to find them.
Don't forget the return journey, that hill is pretty steep. I usually carry all my kit in a Lowe Pro backpack, complete with tripod. Be prepared.
They're a smallish bird and usually keep their distance, a longish lens is best, these were taken with a 500 mm + a 1.4 x extender. Their habitat is shallow fast flowing water in upland areas, often in woodland. Lathkill Dale is ideal. Because of the tree cover and low light levels a flashgun is a handy item to carry, I sometimes use mine with a fresnel flash extender, to throw the light that extra distance.
Note the white coloured eyelids
Taking a peep.
I would say that no later than the latter end of April is the best time to go, before the leaf cover on the trees blocks out all the available light.