Settlement showing up as cracking around openings I would guess. Bay windows are a big , vulnerable "hole" in the front wall, Kat. We would strengthen them very differently in modern building.
Sounds "historic" to me. Probably happened about five minutes after the house was built. Certainly all you need is a general builder. A handyman could do the job, but an experienced builder would be able to diagnose.
He may even use something along the lines of BM's stainless steel ties (post above). Simply scrape out the affected mortar joints, let in the SS tie, then re-point. A few hours work. Easily done from a ladder or tower.