Gently prise off the other bits still stuck, (don't get them mixed up), scrape out the holes a little. dry fit them back into position, to make sure they will sit flush, and then glue the pieces back into place with some PVA. Press firmly into position and gently wipe off any excess adhesive. You may be left with a visible hairline but better than what you have.
Sounds like a plan! Thanks. I've got some 502 PVA.
However, I'm wondering if the screws are making those broken bits stick out and might not allow the pieces to sit flush, and if I should back the screws off but without making them too loose?