But that's still a tiny fraction of the likely pub turnover of £2-400k.

If The Three Stages Heads was running a purely traditional (public domain) tune/song session, then it was up to the musicians to provide the landlord with the information needed to negotiate with PRS - (assuming the landlord actually wanted the session to continue, and wasn't using PRS as an excuse, as has been known to happen elsewhere).

If it was a mixed session - with copyright material being used - then it's right, both morally and legally, for a licence to be required. Just because you choose to call music 'folk' doesn't mean it's actually public domain. You might just as well expect free beer.

It would be better to concentrate on the areas where PRS are out of order, and try to avoid knee-jerk reactions to activities which may in fact be both legally and morally correct.