Phil, based in Essex says it all, and whilst I don't know you I'm sure you're not a knob despite your daft questions. If the guy's a military officer and you don't know his rank and it's a formal occasion call him/ her sir/ ma'am. He/She will let you know if that's OK or if a less formal address would be appropriate. If at a relaxed social function introduce yourself and take it from there. Simple really...one does not need a book on etiquette.Watch out for their spouses however.
Speaking as an ex Sqn Ldr from Essex, Phil, had you met me when I was in you could have called me whatever you felt like. I would have returned the compliment. As you ain't a military dude, you ain't required to observe military bull...only us card carrying service types have to do that. Pretty much same rules for wives too...unless they are in the military it's.. "oi missus", until we are on first name terms. And if said wife/life partner is not comfortable with that we ain't gonna be conversing further anyway, so no sweat.
Like I implied earlier, there are those that think the bull is important and worth taking seriously. And there are/were plenty of us who know that is it a load of crap and just something to take the piss out of. Either way, what the taxpayer wants/needs is a military professional and that's the only thing that I ever took seriously.