I think I would be looking to double up Joist B, rather than removing it -
and then put some new joists from B2 across the stair void. What is the
thickness and level of the concrete floor behind Joist A? Could you bolt A
to the concrete, and thus make it able to support one end of B2?


The concrete floor section is at the same level as the floorboards covering
the joists. I have no idea how thick it is though. I also have no idea how
it's held up so I'm slightly nervous about relying on it for support. It's
a 1900's terraced flat and I didn't even know they had concrete back then!

Andy