Cracked walls after new Upvc windows installed

That's depends if the downstairs bay is load bearing was the downstairs propped up correctly when the bay was taken out and if it was load bearing did they install jacking kits?. All of this may have been done correctly and it's just cracked from the movement witch yes can just be filled in. Hopefully the company you used done there job correctly. Best to keep a eye on it to see if it gets any worse

The ceiling at the ground floor should have been propped in bay window are as the junctions in bay windows are reinforced to support the higher floor opening. Not securing the ground floor opening may lead to disaster.

I would remove the brick or bricks and replace them.

The ceiling should have been propped when removing the old bay window to stop the above dropping. Once old bay is removed and new bay installed they should have used bay pole jacks to support they weight from above once props have been removed. Is done correctly I wouldn’t be to concerned about the cracks. With it being a old property, the motor mix and plaster isn’t as good as it once was and goes crack quite easy due to even little movement. Hopefully the company you used were fensa registered if case of any issues. I would keep an eye on it for the next few days to see if it gets any worse

This is serious issue , some old houses have no load bearing beams , when you most of bay windows remove old windows , upper walls coming down so that’s why cracks on wall, when you remove windows need extra supports , look after walls and ceilings