I agree that "St. James' " or "St. James's" is a matter of debate. It is
considered better practice to omit the additional "s", although I would not
agree that it is grammatically wrong to include it.

Why do I say "better practice"? Because it is The Queen's English and Her
Majesty's Court is known as the "Court of St. James' " and not the Court of
"St. James's". For example, Ambassadors are appointed to "the Court of St.
James' ", not "the Court of St. James's" or even "the Court of St. James".

Marc.