Well, I followed the advice and have gone back to sterile.... made a dilute SB roots bath this morning, left the roots to soak for a few hours, and added approx 3ppm pool shock to the res. I didn't go so far as a full change as it was only changed a couple days ago, and I wanted to make sure it was worth my time/effort before repeating the tedium again. Given how much mass there was to support up top I thought I'd also give the plant more of a fighting chance if I did some premature topping to minimise the amount of plant the roots needed to support.
12 hours later, this seems to have been a great success. Some vitality is slowly coming back up top but more importantly, there's lots of signs of fresh white roots emerging both straight from the pot, but also in and amongst the sludge. It's early signs, but I'm shocked how quickly it's re-rooting. I know it's going to take a week or two to recover, but I feel it's still going to be quicker to rescue this and run with it than start again from seed. (Besides - with the work I've done, I should now have 8 colas and a nice even canopy if the resurrection continues).
Couple of questions... I've got some Cannazym. I'm thinking this could be the quickest way to clear the sludgy roots without damaging the delicate new ones that are emerging. Does this sound like a bad idea?
Secondly, I'm currently running at 260ppm. If I add the zym, this will undoubtedly rise. I'm wondering which is the best idea - to add the zym now, then res change and dial back the nutes in a few days (after all, nutes will still struggle to reach what's left of the plant til the roots are more established), or whether I should dial back the nutes with a fresh res now, then add the zym and re-shock? My basic understanding is that enzymes are proteins as opposed to being 'living' so may be compatible with pool shock, but I have no experience to confirm this.
Thanks to all again for all the advice. Fingers crossed the recovery will continue