Well, it was given as an alternative to a mortar skim plus replaster of
the bottom metre or so, so I took it it was? Or are you recommending
replacing the plaster on *all* outside walls with this method?

IME penetrating damp usually manifests itself in the lower part of the wall.
Most of the rain runs down the wall, and only starts to soak in lower down.

Assuming the original plaster is ok, it's simply adding considerable cost.

If the originasl plaster is ok, it's a pretty good sign you don't have a
damp problem.

If you've cured the cause of penetrating damp by removing piled up soil
etc and made sure the pointing is sound - as you'd have to do anyway -
leaving things to dry out naturally in a habited house would be my advice.

Penetrating damp usually occurs where the mortar has degraded to the point
where it is porous or has developed hairline cracks at the join with the
bricks.