I watched a YouTube video on the Wandle last night, a chap catching monster trout on bread. One had a damaged tail fin but was supposedly a "wild" brown. Any ideas why this would be?
Was it Trouting Time by any chance? If so, from the bloke's fish handling, you can easily see how a tail could get damaged (in a net or due to appalling handling bankside as perfectly demonstrated in the video). Wild tails will also get damaged when the fish are cutting redds, and I have personally witnessed this in the Wandle - a joy to behold!