I have 12mm un-foil-backed plasterboard over a 2" gap, to a 50cm stone wall.
It's a mite chilly, and I'd like to insulate it a bit.

Ripping out the lot, and replacing with plasterboard/celtoex/... is
obviously one option, but is "slightly" disruptive, and involves
a hell of a lot of work, as well as not inconsiderable expense.

Other ways.

I'd need to apply a vapour barrier to the inside - I'm currently thinking of
something like Al foil wallpapered over the walls, and painted over.

I'd then like to drill holes between all the studwork, and simply blow
foam into the wall before replacing the plugs, making good, and putting
the foil on top.

Is there any means of DIYing the foam injection?
Other than buying several dozen bean-bags, and a funnel