Thanks Richard - your answer was very informative.
And thanks Baz - I hadn't even given PAT a thought.

So, for four musicians (we're not likely ever to have more) we need four mics plus the radio mic for Mo (the caller).   Perhaps, with this number, the sound won't be muddy enough for us to worry unduly.

The recorder will be difficult to mic anyway but that's the musician who isn't likely to be there very often so perhaps it isn't something we should lose any sleep over.

Neither of my melodeons is pluggable although the Saltarelle has velcro stuck on both sides which suggests that it's previous owner used two mics with it.

I have to admit that I'm seriously wondering whether we need amplification. My late brother in law had a dance band with a line up of piano (said brother-in-law), saxophone or clarinet, double-bass and, occasionally, a guitar. They played in some very big halls for dances with 150-200 people (I know cos I was there) and were never, ever amplified. It didn't seem to be a problem so what has changed that makes amplification so necessary these days? A melodeon, two concertinas and a recorder can make a hell of a lot of noise. Is it just that people expect amplified music and feel cheated if they don't get it?