i have an old house..80 odd years.. the plaster is pretty caput and
theres no cavity wall. what i want to do is insulate the outer walls as
im renovating it. i figured that if i could get some insulation, then
put plasterboard over it then id have new walls and an insulated house.

what insulation should i use?

I would use Kingspan K17. The plasterboard is already bonded to the
insulation, so you don't need to worry about that!

The insulation can be dotted and dabbed to the existing wall surface, if it
is suitable. If not, use K18.

http://www.insulation.kingspan.com/newdiv/k17.htm
http://www.insulation.kingspan.com/newdiv/k18.htm

Christian.