FWIW This concerns a flat in the middle storey of a small three storey
block, which was built in the 1970s. I won't have access to the floor
above, as that's obviously someone else's flat.

I would expect a 1970's flat to have decent fire protection between
flats/floors - more than plasterboard/joists/floorboards. Are you sure
there isn't concrete in there somewhere?