The one firm I've dealt with in your neck of the woods, in the past, is Portwood Timber, who used to be OK. Looking them up I see they are now part of Illingworth Ingham, a large chain. I've been on jobs where Illingworth's have supplied materials, too, and had no issues with themI am near Stockport, I do have local timber merchants.
Yes. Just pin some 50mm wide rips of hardboard onto the tops of the old joistsI was thinking of getting 18mm boards and packing with 3mm hardboard to get to something like the old board thickness which was about 22mm.
Does that sound feasible?
No idea where you are in the UK, @^woody^, but up here in the NW it's been rain for a week and high humidity for longer, meaning that installed boards are tending to shrink after installation rather than swellIn a house in this weather, they may want two weeks to fully acclimatise.