So, daily update as I'm now quite enjoying this forum. And maybe the thread could be helpful to another noob like me one day.
Second flush was left to run on the normal flood and drain cycle last night. This evening the res was reading 0.5 EC. I've dumped that water, refilled with plain tap water with 0.8ml per litre of the 12% h2o2 (actually slightly below the instructed amount on the bottle, but I've aimed conservative). After 30 mins circulating that water read 0.3 EC. I've now dumped that and am doing one more fresh flush cycle of 20 mins (plain water plus h202).
The plan from here is to drain that res. Then refill with a lighter feed (about 1.2 EC) a touch of calmag and 0.4ml/litre of h2o2. I'm then going to have to leave it for 3 days and hope for the best. Mrs birthday tomorrow and I've agreed to take her out, followed by a weekend away at her friends. If there's anything blatantly stupid in this plan please do chip in.
Since yesterday the leaf damage has increased very marginally, but I know things happen quickly in hydro and I feel like I'm making progress here. Buds still looking lovely so I'm optimistic of a decent harvest in a few weeks time.