When The Times had this story today there was reference to the lack of a full stop in St Mary's Street. I did read a rule once, but I can't remember where, that an abbreviation should have a full stop unless the last letter of the actual word occurs at the end of the abbreviation. Thus St Mary's Street is correct because Saint ends with t. However, Regent St. is also correct because that t is the second letter of street. So there is a nice distinction.
Mr Smith is correct (misteR), as is Mrs Smith (MistresS), though one often sees a full stop inserted.
I am glad to say that The Times leader was fully supportive of the residents.