You can continue adding a small amount of bicarb with your weekly water changes to maintain the kH in the tank, or if the fish you have in mind need harder water than you have, you can dose with a product such as JBL Aquadur to raise both kH and gH as required.

Well done!
If it were me, I'd choose soft-water fish.
My water is very similar to yours. I do have a piece of Tufa/limestone rock in there but it doesn't make much/any difference - it's probably become inert over time or only increases PH if water is poured directly on top of it during water changes to 'activate' it (ie white powder released).
I generally opt to keep rasboras and tetras (lots of different species to choose from) which are ideally suited to water like this, as are corydoras. Let us know if you'd like some suggestions but, as a starter for ten / rough indicator, you probably could keep a shoal (8-10) of either harlequin rasboras or cardinal or ember tetras, a shoal of 8-10 panda corydoras and a pair or small group of honey or sparkling gourami. Hope that helps.

Thanks both 😊
My daughter has had her heart set on neon tetras since the beginning so that’s good news! We were thinking a group of these, as well as some corydoras and otos - and thanks, the numbers are useful indicators too. Will probably have to leave getting the otos until we have built up more algae for them though. Will look into the gouramis.
So, are these all ok with a ph of 6.6 and KH of 1 drop? Do we just leave it alone and all will be ok or do we add some bicarbonate of soda as Fishlady suggests anyway, and if so, how much/to what KH? Someone suggested adding broken oyster shells to substrate - is this better or needed? Or will it do a similar job to your limestone as in not much?! Our KH does seem to be really low (tank and tap same).