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

Have you asked yout tame structural engineer for advice?

Not on this one (yet) - but if he comes out again to measure up for the loft
I might pick his brains a bit more...

Andy