Thanks all. Love the knowledge and helpfulness on here!
Given above, I strongly suspect the problem was the bicarb solution.
I have the extra LED light now - which does add a reasonable boost, and has colour modes as a night light.
Seachem Flourish (which I already have) is derived from forrous gluconate and potassium choride (albeit it doesn't seem to have any potassium in it - only soluble potash - K20). Not sure whether I should be getting some Flourish Potassium (or similar) as well - if this is regularly deficient. The Seacm FLourish is described as a comprehensive supplement, yet seems to be micro nutrients and trace elements - and suggests on the that for macro element (NPK) fertilisation, I may need other supplements. However, given the Topica aquasoil under the sand, and the types of plants I have, I'm (perhaps naively) hoping further supplementation won't be necessary.
A handful of the vallis are regrowing (I think from that batch that were only soaked for minutes rather than a day).
The anubias looks a bit spongey - a few of the leaves came off/stalks went mushy. Hopefully it'll recover...
I'm planning to replace the plants (without the bicarb dip this time) and we'll see how they get on. And add some frog bit too.
Will report back if any issues!