Musclemech,
Don't overlook the obvious - you and Mrs MM might simply have been invited for your quick wit and repartee. The format of the invite tells you one of two things - it's a "no civvies" do only, or it's the only way the dear chap knows to have a bash.
Either way, you and your wife are not being invited for your knowledge of Debrett's or JSP 101, so get the basics right by all means, but just go and enjoy yourselves. Mrs Willoughby-Smythe will be delighted that you came and enjoyed yourself, only the proles would be rude enough to laugh at your home knitted Thomas the Tank Engine sweater.
One tip with "mixed' functions - do wean yourself off acronyms and abbreviations before you get there. The anecdote about the Pygmy and the Gynecologist will be funnier without all the staccato alphabet soup we all tend to use these days. People who don't know what an ATTC BPS Double overhead knurled flange-bracket is may feel excluded.