Hi Alex
Apologies for the long cold silence... Life has really got in the way lately... Did you ever solve this?
I'd start by looking for the reason it slopes off like that. What year was the house build and what part of the country is it in?
Was it sloped before taking up all the existing coverings or it was only noticed once the coverings were removed?
One thing I would be inclined to check is the neighbours houses... Do they have the same thing?
(See what I did there? Inclined.., ahhh, I'll get my coat... Taxi...)
There could, although unlikely, be a solid reason for it, which a little research may reveal. But if it's isolated to just you and nothing unfolds, then it could just be a single bit of bad luck.
The best way around it is to source the exact same wood, in width, and simply make a shim to sit on top of the joist, but this would then create another problem, as I assume, your skirting boards are all installed at the height of the floor, so would also need raising up.
Did you get any pictures of it?