I would go with what you see. The marriage cert is a scanned image of original on Ancestry. Also you know that James Brown was not born Grundy.
He was a nurse child in 1891, his foster mother Ann married Peter Grundy in 1892 he then stayed with the family and took on the Grundy surname.
Martha Ann Foster was the illegitimate child of Ann Foster, baptism and GRO birth reg verifies. She is Martha Grundy in 1901. Married as Martha A Grundy in 1910.
All this can be verified by census, GRO birth reg around birthdate and pob. There is no birth reg or bapt to connect James to Ann Foster. Also online trees have some or same evidence. I know that online trees can have misleading info, but some get it right.
I do not have anymore suggestions to help, sorry, rather than what information is given on thread by all.
Best wishes. Good luck with your search
Cas