@Olly398 did yours look something like this? In-filled with brick dust and rubble... I'm guessing I'm going to want to clear this out and extend the joists for underfloor ventilation and to support the new boards.
That's pretty much what both of ours looked like. I just dug all the rubble out from underneath and took a sledgehammer to them, then bricked up the hole left behind. Took far longer than it ought to have done.
I dug out the rest of the rubble last night, and removed the bricks of the constructional hearth. I plan on fitting a small joist between the chimney pillars, then two from that over to the existing floor.
@Olly398 did yours look something like this? In-filled with brick dust and rubble... I'm guessing I'm going to want to clear this out and extend the joists for underfloor ventilation and to support the new boards.