Thanks Andy, you've confirmed my gut feeling about losses, and also
that one episode of tripping over a pipe and putting a foot through
the ceiling will take a hell of a time to balance against the extra 8"
insulation.

Consider using Kingspan or Celotex. Although more expensive that rockwool,
100mm will give a lot more insulation that rockwool. It is also longer
lasting and less prone to sagging and soaking, as it comes in solid blocks
looking a bit like expanded polystyrene. You can board straight over the
top.

Christian.