Big gap around window

[Andrewd8323]

so window is the correct size

I guess you are having plastered walls internally?

in which case, fill gap with foam, use insulated plasterboard on the reveals to take up the gap

by the way there appears to be no damp proof arrangement on the reveals -I suggest some DPC vertically against the brickwork but bent around onto the window frame, then put plasterboard on, then once plastered cut back DPC

Thanks for your message - going to try and do this today! Am I right in thinking I should use the foam first then fit the DPC?