Long time no update here, so I thought I'd let you know what is happening at the moment.
All the fish in the temporary tank are doing well, are settled, and feeding well.
Over the past couple of days I've removed the rest of the sand from the main tank, and removed the river manifold. The manifold was hurled into the back garden in disgust, and the removal of the associated tubing & pump nearly made me sick. I've cleaned the tubing, and pump. It was all quite grim, and I found a dead assassin snail in the pump
Although I had filter sponge on the intake, somehow the snail had got in, and probably explains the smell when I took everything apart. I don't know if this would have had any impact on the river tank disaster last year.
@Andy The Minion has been kind enough to provide some diagrams of a different set up for the pump/manifold, and now that the tank is empty I'm going to spend some time trying a simplified set up, based on a combination of the information AtM provided, and some ideas on how I can have it set up so I can remove the pump hoses/intake/etc. for cleaning on a regular basis. This means I won't be rebuilding the manifold I previously had, which sat under the sand, but it may take me a while to work out exactly what I can do. The ideas are floating around in my head, I just need to have a few practice sessions in the tank while it's empty. I plan to use the pump to create flow near the bottom of the tank, then 2 external filters for the middle and surface flow.
In the past few months I've been purchasing more of the yangste rock from Aquarium gardens. I saw it while the main tank was still set up, bought a couple of pieces and put them in. Unfortunately this made the other stones I had look quite dull and unnatural, so as part of my always steep learning curve I'm using the disaster last year as an opportunity to try to make the river tank better.
Without a manifold under the substrate I should also be able to get away with a lower level of sand, which should help to avoid any build up of anaerobic bacteria, which always concerned me.
I still have the bucephalandra attached to wood, and I'll probably just use root tabs of the echinodorus martii major when that goes back in.
Time for a glass of wine while I contemplate the pump/simplified manifold set up.