Colin,
I haven't got back as far as 1650's yet but my Gwythers are also from Monkton area. My earliest one is a Thomas GWYTHER married to Lettice DAVIES 12 Jul 1755 in Rhoscrowther. Thomas was born in Monkton. I am descended from his son Peter who married Rebecca MORCE. You can contact me on
ewa06271@bigpond.net.au and like you also in Australia.
I have a good researcher in Pembroke I can put you in touch with who should be able to help you.
All the Gwythers in the Monkton area are related.
Cheers Ellenor