Given that it's render above you could build a timber frame (insulated), clad with cement board and rendered to match existing. It would probably look better than mismatched bricks and/or brickwork where the adjacent houses don't have any.
Having thought about it long enough I have decided to go with this option as it looks like the easiest option for me and I know I’ll be sacrificing natural light into the room.
For the timber size would treated 2 by 4 be enough or should I be going for something more substantial in size?
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