Thank you, Jen.
It is indeed persistence. I think this one has had all of us totally gripped (obsessed?)
Davy, yes, it won't be Martha junior. Is there another Martha somewhere in the family? It could all be a fantastic coincidence, but then what happened to the widowed Martha after Jacob died?
What we do know is that a Martha Coulthard married a James Brown in 1918. Which may be one coincidence too many? Because there weren't lots and lots of people with that name.
Unless someone has a brainwave, it may come down to getting that marriage certificate.
The other thing is - if it is your Martha, then either she was a bigamist, or William Turner died before 1913!
John