Mark, I feel your pain. The Chess below the now Croxley hall fishery was on my West Hampstead ticket, Sabey's pool etc. down to the Colne. Was told about Trout upstream , St Georges park area. Aged about 16 I ventured forth and caught my first ever trout running a freelined worm under the fence going into the British Fresian society grounds. A future visit I met with the "keeper" in the park who showed me the one surefire 'lie' in the park, had a few over the years including a few Rainbows. The water was always crystal clear with a distinct lack of any weed whatsoever. Ankle deep maximum. That would have been the 70's and I went into a game fishing only for some years. Always kept my eye on the Chess though. Must have been about 93/4 I began to see weed growing, the level increasing, obviously, and some decent fly hatches coming off, this all apparently because of the Chesham sewage works improvements Healthy growths of Rannunculus established themselves and some phenomenal hatches of mayfiles happened. Water was now knee deep and in Mayfly season it was without doubt the best wild brown trout chalkstream fishing in the country. One of the last days I remember, I decided to count, I gave up at around 30, notwithstanding the juvenile tail slappers getting hooked in the tail attempting to drown the huge mayflies. Fish averaged about 1 1/4 lb. best one that day about 3 1/2 lb.
Sadly the river was devoid of weed, flies or fish the next visit, but was low enough that the doctor at Loudwater wouldn't get his riverside garden damp again.
The EA had done this. Destroyed an entire ecosystem.
They've managed to cock it up on several fronts since, fish kills, ignored warnings about bank breaches, they are worse than useless.