A question for the legal minded. Can said King Rollo (we will asume he is a PRS members) or any of the unpaid participants in the music making (we will assume they are not members) - that has been prevented in these Sam Smiths pubs - make a case for the unlawful restriction of their freedom of expression?

Who would the claim be against?

The pub has a right to a choice to first allow/provide music. And to obtain or not to obtain the PRS/PPL etc licence. So perhaps not the pub.

But PRS/PPL etc are publicly giving the impression to all-and-sundry that no music at all - can now take place in these premises without this licence. So the right of freedom of expression of a non-PRS members playing their own non-published material is currently being prevented in these pubs - where as far as I can see - there are no grounds to prevent this freedom of expression.