Harris wrote:
I've got a problem with a room which has very old plaster walls, 3 of
the walls are brick with plaster, which I'm not too worried about,
I've patched some holes and the whole lot feels pretty strong.

The forth wall however is just wooden slates with plaster, and no
mater what I do with it, it just feels weak.. (If I push the middle
of the wall it bows a bit)

I'm thinking I could fix plaster board to the wall to give it a bit of
strength (not sure just how much strength it would give it?), is this
a good idea? If so, how do I fix plaster board to a weak wall?
There will be vertical studs spaced around 2' apart. You can detect the
studs easily because of all the nails holding the horizontal lathes.
Simply screw the plasterboard to these studs using drywall screws which will
have to long enough to go through the old lathe & plaster which will be
about 1" thick.
I saw this done recently but the plaster was knocked off the lathes first
because it was "blown".
The lathes were left in place and the plasterboard screwed on top. The
result was good, firmer than a modern stud wall..