Roland Perry wrote:

I'm tying to get a feel for whether this approach has any merit before I
start getting some costings.

For an existing building, you need to achieve a U value for a new wall
of 0.28 or better, have a poke around with a calculator, e.g.

https://www.uvalue-calculator.co.uk/calculator/walls/timber%20frame/insulation%20between%20timber%20studs%20with%20(in sulated)%20plasterboard/89mm/brick/standard/32.5/50/

An 89mm timber-framed wall, with 50mm PIR insulation between studs
(obviously you could go up to the full stud depth to improve it),
external brick skin and 32.5mm PIR backed plasterboard inside can meet
it, not any less thick than an insulated brick/block cavity wall though ...