"Mr. Amsassador" is certainly not appropriate.

just say 'oiroight mate? how's it swingin?' - no need to be over formal after all...
or 'your excellency' would do it if you want to show off

@Volker: Fürs Vortragsdolmetschen haben sie uns während des Studiums eingehämmert, NIEMALS Mr/Mrs in der Anrede zu verwenden, wenn es um die Funktion der Person geht, also immer Chancellor und nicht Mr Chancellor, Secretary General und nicht Mr Secretary General. Vielleicht wäre es möglich zu sagen "Ambassador Torry", hab ich aber versäumt zu fragen. Muttersprachler müssten hier helfen können.

Wenn du ihn nur zum Händeschütteln begrüßen willst, wäre wohl die Sir-Variante am besten, aber auch hier bin ich nicht sicher - ich dachte immer, es ist nur möglich Sir+Vorname zu sagen. Native speakers, please help!

Diplomats:
Although "honorable" is applicable for American ambassadors, the terminology for foreign diplomats differs. A foreign ambassador to the United States should be addressed by "his excellency" or "her excellency." The proper way to address a British ambassador is "His Excellency, the Right Honorable Justin Case."

http://www.editpros.com/news0405.html

Nur rasch zum Gebrauch von "Sir":
- ANREDE entweder nur "Sir" oder - bei einem knight - "Sir + Vorname".
- Sonst "Sir + Vorname" oder "Sir + Vorname Nachname", also "Sir Paul" oder "Sir Paul McCartney"

@all: Thanks a million!!!

I think I'll try "matey", "dude" or "homey"! Just kidding, I think I won't get arrested for using "Sir Peter", sounds good to me!

@BBee: I think an ambassador should be addressed "Your excellency". "His/her excellency" is used when talking about the ambassador with a third person.

Ambassador (British)
CORRESPONDENCE: On the envelope, (When in the country to which he/she is accredited only), "His/Her Excellency [preceding all other ranks and titles], HM Ambassador to_____". Commencement, "Sir/Madam" or socially according to rank. Conclusion, "I have the honour to be Sir/Madam, Your Excellency's obedient servant".
PERSONAL ADDRESS: "Your Excellency".

Ambassador's Wife
She is not entitled to the style "Her Excellency" and is referred to and addressed by name or conversationally as the Ambassadress.

From: http://www.debretts.co.uk/etiquette/correct_f...

Thanks again!!!