I can’t find a source for the rule on the web, but I now know more than I ever wanted about the British peerage. WikiAnswers has this, but without a reference:
http://wiki.answers.com/Q/How_do_you_address_a_former_ambassador
I tried “courtesy titles” and “styles of address” - even with quote marks the results are primarily garbage. I remember Mrs. Cates covering this in 4th (or 5th - I had her two years in a row) grade English class, but since I can’t document it, I won’t argue with your usage.
I only asked because I couldn’t find anything either. Also, a search for “former ambassador” turns up lots of hits.
It is in accepted common usage, unless you are Governor of Alaska, then you are addressed as Mayor, or Wasilla Hillbilly.
Get over it common rules change by default, else why would gay be the acceptable substitute for Homosexual?
Who in their right mind would have thought there would have to be a constitutional amendment to define marriage, since it had been in common usage for a few thousand years.