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.
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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.
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Note the white coloured eyelids
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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.