ESpecially in a context where it was essentially more like a practice session, as you suggested Shambles - a chance for musicians to find whether they like playing together - in a situation where the music itself is really incidental, and therefore arguably excluded from the restriction.

I think continuing that dialogue with the MU would be well worth it. All the MU people I've ever chatted with at festivals have seemed very helpful They seem to see amateur folk musicians as allies rather than as any kind of enemy - and there are an enormous number of instruments around with MU stickers on the case most places folkies congregate.