The upper Manifold in Derbyshire. Small stream, but with some big holes.
I've told this story before.................

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I fished there several times a few years ago in low water and bright conditions. I was spectacularly unsuccessful and was wandering, as you do, looking for a rising fish I could cast to without being seen.
I found a small deep run and from the overhanging bank and behind bushes catapult-casted a #16 GRHE into it, vertically dowwards. As the fly was sinking this grayling rose from the depths, sucked in my fly, ejected it and sunk back down.
It was the size of a submarine

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Perhaps not, but it had to be 5lbs +.
DB