Good ideas, and challenging, but it's a shame that at least four red herrings create pairs that can be swapped between groups.Nugget can be swapped with Tooth, Barrel with Letter. There's only supposed to be one workable solution to the grid, otherwise we have to be mind-readers to solve it, and rather than 'fiendish' the wall becomes 'insoluble'. I'd suggest editing this one. Put Letter in Barrel's place and Nugget in Tooth's place, then replace them in the 'double letter' group, with Utter and something that doesn't fit into the other groups.