Some advice regarding a hole in the bricks!

[Grenvilledodger]

Hello everyone. I am a bit of a TOTAL novice but hoping to learn a few things on here!
I have a Victorian Terrace with a gas meter cupboard next to the front door. We have had a load of issues with damp (recently fixed by a professional). However, the front door frame has rotted (planning to have a new door fitted) to the point where you can see daylight through the frame. There was chip board (poor choice of wood) inside the box which has more or less disintegrated now. There are pieces of brick missing too - they have disappeared somewhere in time prior to my moving here.
I want to fix the problem brickwork/hole. I expect the new door frame will largely solve the issue, but I will need to do something about the missing bricks etc.
How would you go about filling in the gap in the pictures? Would expanding foam be any use? Or some type of cement? Or bricks cut to size with cement? I would probably seek to put some plywood over it (inside the gas box) for aesthetic purposes, once complete. But I want to stop the wind and rain from coming in first through the hole! Cheers all in advance.