Not managed a bullhead or stone loach yet, but gudgeon and minnows yeah. Bleak will throw themselves at pretty much anything but try catching a minnow off the top, that's some frustrating sport for ya
Fishing the duo method of a small nymph under a dry is a good start, though you'll need something small and heavy to get down to those gudgeon if they're in some flow(as usual). Flies need not be fancy just small, try beadhead PTN's or GRHE's and go from there.....
This monster took a 18's Tungy PTN

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Cheers,
Graham