A very British question

Greetings, fellow writing fellows! :)

A friend of mine, who is in the midst of composing a semi-fantastical golfing tale, has put to me the following question...

"I have a character in my story who is the vicar of Warmsly (http://www.slightlytrue.com/writing/Choir-Practice.html) and he is playing golf with Lady Fiona Hardcastle. This is a social occasion, and they play together often. How does he address her Should he call her Fiona? How should she address him? Any thoughts?"

So...

Any thoughts?