If you referred to Prince William as "Cambridge" I think it would not be viewed as appropriate -- "The Duke of Cambridge" is what you should use!!
Best to use either a title or First and Surname together -- then you will not be wrong.
R
Dear Lord Plus Faw of Kilgibbon,
I won't ask the reasons but I will follow your advice. This is a minefield. In my profession, everyone is called by their Surnames. I shall try to use Mr. from now on.
Thank you and Mr. Neilda for your patience and explanation. The public rebuke was a surprise.
D
