Wall Insulation

Hi all,
Just bought an old farmhouse and haver started the long task of bringing it up to scratch. Two of the rooms seemed reasonably ok and thought they would just need oaint and flooring. However after moving a couple of sockets I have found that there is no insulation behind the plaster board. There is a gap of 4-6 inches and a nice cool breeze! Any suggestions on how to insulate with minimum damage to the internal wall? A neighbour suggested cutting a 2 foot panel out of the wall all around the rooms and stuffing rock wool up and down to fill the void. Would this work?
Just to add a bit more information. There are no signs of damp so doesn't seem to be a concern at the moment, maybe due to the draught! Also the walls are solid stone about 24 inches thick so there is no cavity to fill. In the attic space there is two inch boards of white aeroboard in between the rafters. would I be better off to pull them out, leave them in alone or add something like rockwool on top?

This has come up before. Check out Here.
I got my house done through premier insulations and started a craze in our estate - all cavity wall built houses without a square inch of insulation. found it to be the quickest and cheapest solution that stands up to building regs (as I understand it).

Cheers for that, lot of very useful info. Will be on this site a lot in the next couple of years. Old extension to be pulled down, new one to be built, open fires to be changed to stoves, maybe a bit of eco energy, etc.