Have you the year of birth for James? James Walker is a very common name, I notice no middle initial on the marriage entry.
I had a quick look for Wills and there are a few,but only going up to the 1940s.
Believe me- after 35 years at this "hobby" nothing comes easy. I still have huge blanks and brick walls, you can count on the fact he will not be still alive....no one lives that long!!
You say his grandaughter remembers them moving to Mansfield,does she remember how old she was when this happened. Was she perhaps already married?Or at school. My mother STILL comes out with snippets of family info. When I say you never told me that before, she says I didn't think it was important...AAAGGH