Originally Posted by Monza
I was only speaking of pronunciation. St James's is the correct possessive for a single St James. If the park were dedicated to all the saints of that name it would be trickier: St James' would be technically correct but confusing. St Jameses' would make a degree of sense but be unwieldy.
It's easier with surnames. Marlon Wayans's family, the Wayans, live in the Wayans' stately home.