Well I've come to the conclusion that my Symmons's were keeping their surname when all around them were still changing theirs, because they were related to the gentry, perhaps through illigitimacy or through a black sheep!

What I know about the richer Symmons families is not very inspiring (e.g.John Symmons MP never once spoke in the House and squandered the family fortune), but a connection would make the genealogy so much easier. I'll keep looking for a link.