Hello Adam,
I am also descended from John via John's daughter, Sarah.
Cambridgeshire's non-conformist records have not revealed anything as yet, regarding James, but I know they still have other records to make available yet. I have a cousin who has been researching James for years, has been to the records office, looked at all the records available at the time, but with no luck.
Hopefully we will have better luck in the future.
All the best,
Rosemarie