As the price of heating has increased, I have started to think about Cavity
wall insulation again.
I have previously been put off the idea due I always thought that you needed
the air gap to prevent damp,
therefore filling the gap with rock wool may cause damp.

Does any body have any ideas as to the problems that can occur ?

As it does seem a no brainer as it will only cost about 200 pounds and this
can be saved in one year.