Single skin brick bay joining on to external wall of cavity how to fill gap

[trimattrdg]

On removing a bay window seat and some plywood from the bay window ( which effectively joined the cavity’s) on 1920's semi I have exposed the single skim brick which ties into external brick face of a cavity wall.
This leave an open cavity of approx 5cm that you can look down on either side of the bay.

I'm looking to insulate the bay with wood fibre and continue this wood fibre across the inside of external wall.

Is it ok to close off this cavity gap from the room ( if so what options are there for closing the gap ?)

 

[trimattrdg]

 

[aockenden]

I’ve recently used sheepswool coated in a sloppy slurry of clay and lime to pack the drafty, narrow, hard to reach gap between a joist and an exterior stone wall (pic of the gap). When it’s dampened with the slurry it becomes easier to compress and solidly pack into large gaps like yours. It then dries stiff and firm. Just a thought.