Hello, Hugh,
It is a long time since you posted. However, I think we may have a common ancestor in John Kerr of Slamannan. I have hit difficulties in tracing both Robert Kerr and Margaret McAvoy (?), the parents of John. I noticed there was a John McCann on the documents for John and Martha's wedding but beyond that struggling to find a connection. If you have found more I would love to hear,
Regards Liz