Hang on.
St. Thomas' is spelt that way because it is dedicated to TWO St. Thomas's.
Thomas is plural so it's St. Thomas' - see?

"Graham J" wrote in message
...
Park, just as I would say princess's or Thomas's. However many would find
Thomas's ugly and just use Thomas' (as in St Thomas' Hospital). Sometimes
I
do myself and I will sometimes say St James' Park when it trips off the
tongue better.