I have been lumbered with a porch on our ground floor flat with two
external masonry walls (one course of bricks).

To make the room habitable I need to install floor, wall, and probably
roof insulation (flat, cold roof).

So far I have applied a bitumenous damp proofing to the concrete slab,
and lined these walls with a treated timber frame 100mm thickness (with
a gap between it an the external walls varying from 10mm to 100mm due
to the curved wall).

My question is- how can I best insulate these external walls? I was
thinking masonry DPM timber frame with insulation inserted in
between timbers. Will this be adequate for warmth and the building
inspector?

Rockwool cavity seems to be a good balance between quality and value
although I'm not sure it is suitable for insertion between the timbers
like I plan.

Please provide some guidance.

Many thanks

Daniel