Peter Purr wrote:
I need to hang something from a single point on a ceiling. The load
will be about 300Kg max, after allowing a suitable safety factor
(actual load closer to 100Kg).

Any ideas or caveats?

Yes, forget it!

I meant to look for a joist tonight in an out of the way location
(probably a cupboard), make a hole in the ceiling there & have a prod
about, as I'm not sure how much variation there is in possible
structure, but the battery's flat in my stud detector, so that'll have
to wait until tomorrow.

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.

You can't do it without using steel, and to do that you'd have to do it from
above, even then it's not reccomended....the load would have to be spread
over 3 or 4 joists, you'd have to place a steel plate on top of the joists
(chisel the tops of the joists out to accomodate the steel plate under the
floorboards) and bolt a long threaded bar down to hang your 'whatever'
on...the nut would have to be below the level of the floorboards too.

What are you going to hang from a ceiling that weighs 200lb+?