The Natural Philosopher wrote:

My real thrust was at those people who have bought a nice solid
(literally) walled Victorian property, find it chilly, and costing a lot
to make snug, and instantly go out and fit DG.

snip!

I'll concur with the gist of this message. In our case, our (modern)
house already had insulation in the cavity (and I'm not going round
adding an inside skin of more insulation and plasterboard). I'd already
topped up the loft insulation to about 12-14". It's a solid concrete
floor. The *last* thing to do was the DG.

That was done primarily to improve comfort levels (less condensation,
less draughts coming off the cold SG pane, more even temperatures) and
the SG windows needed maintenance....

J