You need to fit as much as you dare between the uprights, whilst leaving a wee air gap on the cold side and then as thick as you dare (without compromising the window board) ACROSS the studs - prior to fixing the plasterboard.
Hi all, decided to tackle this myself. Wasn't expecting straight to brick wall. No going back now, but thinking I'll need to build inwards now with small gap from brick > insulation board > plasterboard?
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