Horatio B.....nothing wrong with "mate" aslong as it's used in context. Using Sir and Madame is good etiquette to some but with others it is deemed too formal and even awkward. By the same token, calling a gentlemen in his 50's "mate" is not appropriate.
I certainly don't call everyone Sir and Madame and I even use mate but it I use them both in an appropriate context and that is the important thing to remember. Addressing someone in a rowdy stag party as mate is, IMO, wholly acceptable whilst calling someone older Sir and Madame is a great and respectful touch.