Congrats on the initiative, and I'd find it fascinating reimagining the windows as they once were. As for the crack and the works required to make good I'll let people more experienced/in the know comment.
Wow. Those are old bricks. 1600s maybe? Probably not much in the way of foundations and the building has settled a bit. That movement over that much time is really slow so easiest to just fill the gap with lime mortar. Could maybe move some bricks to make it look pretty if you want to leave the bricks exposed.
Thanks Sgking and Malcolm. We think the cottage is circa 1700. It's the cottage in the middle of the photograph with the greenery on either side of the front door. Originally a Georgian cottage, with a Victorian extension followed by one in the 1960s.
The intention was to leave the bricks exposed and repoint on the side elevation. It all depends how damaged the remaining brick faces are. Now that we have found the windows, I am hoping we can reinstate them.
If your soil has a high clay content and you have shallow/no foundations there is a chance the crack will at least partially close up again after a good spell of rain