Don't forget the huge difference between the infrastructure of English folk (I know little about the Scottish scene) and Irish sessions, especially back in the day. The session is a place where everybody knows what they've come to do, and do it together. It's a specialist musical platform. Proficiency is appreciated, but egotism can all to easily wreck a session. The musicians get warmed up and into the swing over the first few tune sets. In pop up sessions (like at festivals) the musicians are getting to know each others' tastes and capabilities, and the tenor (hopefully not banjo*) of the session develops. It has its restrictions; G, D and related keys predominate. Though there was a fashion for fiddlers to tune up a semitone about 30 years ago. I never worked out whether that was to line up with sharp flutes or to spite the box players.

I think the CD "Music At Matt Molloy’s" captured this quite nicely, starting tentatively, working up to a full head of steam, then a musicians' break and a bit of singing, the whole thing getting a bit drop taken, but finally rallying for a great send- off.

The folk club was (is?) a place where people came to do their own thing, serially, and can cover a huge range of types of material, delivery styles, keys etc. Instrumental music is often difficult to fit in this, each set starts from cold. And you don't know, going into a strange club, what the local rules are. I remember clubs where songs deemed to be "music hall" were streng verboten.

Add to this the antipathy to competence some folk clubs affected back then, the attitude that "ceremonial" music should only be performed in full context, and the sometimes poor quality of records available for learning tunes from (musicians "collected" in old age and clearly past their best).

BTW I'm a Believer works beautifully on a McCann Duet concertina. I'll play it for you sometime, if you're not quick enough to stop me.

* I recall with horror a session where I ended up stuck in a crowd of 6 tenor banjos. It was like working in a shipyard.