Clive (Clive ) gurgled happily, sounding much like
they were saying:

I have a room, with a gently (20 degree?) sloping ceiling,
following the roof line, in which I wish to fit LV ceiling lights. I'm
quite comfortable with the slope. I need advice on how to create a
ceiling to which to attach the lights.

The ceiling plaster seems to me to be the original, probably on
laths. There are some major cracks in the plaster but when I tap it very
few areas have a hollow sound. By pushing on areas of the plaster it
doesn't seem the plaster has come away from the laths.

So, should I make a frame attached to the walls and with 2 by 1
timbers on to which to nail the plaster board, leaving the original
ceiling in place and creating the void for insulation and the light
fittings? Or should I take the current ceiling down, exposing the laths,
then nail 2 by 1 (or whatever) to the exposed roof beams then nail the
plaster board to that? My thinking about taking the current ceiling down
is although it seems sound it may "detach" from the laths in future
(even though if it is original it has been in place for 100 years !).

Pull it down. It'll make a ****ing *horrific* mess whilst you do it, but
it's the best opportunity you'll ever get to get some insulation in
behind that plaster.

We did this earlier in the year in an upstairs bedroom - again, Victorian
house, in the roof slope. The plaster was, like yours, cracking slightly,
but seemed solid. It really didn't take much to bring it down in sheets.
There was _nothing_ behind it until the roof tiles. Not even tar paper.
Except for some long-empty wasp's nests.

75mm insulation went in there, between the rafters, then plasterboarded
and skimmed.

Very well worth the effort.