Just as an aside, and after reading all of the posts in this thread...

I hope that you have adequate Public Liability Insurance!

If you start tinkering with the removal of these joists (even though you
take adequate precautions etc.) and the ceiling caves in on the downstairs
flat (hopefully while the people are out...) there'll be HELL to pay! If
they're in at the time, you might have to pay the next-of-kin!!!

Also, as someone else pointed out - but I don't think you realised his
point, are you the Freeholder of your flat? You can be the owner but not own
the Freehold - in which case you are a Leaseholder. If you are a Leaseholder
you will need to obtain the permission of the Lessor to make the alterations
(which you prolly won't get), and if you do there could be a clause to "make
good" before you sell - which means putting it back the way it was
originally! That may prove difficult if you've already sold the staircase
space.

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