Hi Stuart,
I don't know whether you have gotten any further since the posts that are here, but Pam and I have been able to confirm Margaret Froster as the wife of William Rettie/Raittie (b.July 6, 1678), and that his father was John Raittie.
You might want to talk to Pam about further info as we also found several more children. She has copies of the original baptism records. I believe she found them on scotlandspeople.
I'm away for a month, but will continue to dig.
Sam/Val