The difficulty is guaranteeing that the all music to be performed is in fact non-copyright. Many songs and tunes which are thought, or assumed, to be "traditional" are in fact copyright. Many more genuine "anon" have copyrights registered for arrangements, and the PRS won't know the difference - if in doubt they will put the onus on you to prove that the version played is non-copyright.

If this session comes out of a class you teach you may have some control over the repertoire, but where participants are able to choose their own material you will not know in advance what they are going to play.

Responsibility for getting a PRS licence is the venue's, not the musicians' or the session organiser's. The pub may very well have one already, but make sure it covers live music and not just recorded music. If the landlord properly understands his liabilities he will probably not want to rely on your assurances that it will all be non-copyright and will want the correct PRS licence to cover himself. Whether he expects you to pay for it or bears the cost himself is for negotiation!

The current PRS tariff can be found here:

PRS concert venue tariffs

It looks to me as if 3.1.3 is the appropriate one, £9.90 per session. Or you could try 3.1.7 at £86.89 for a year, although whether a regular session can be said to be spontaneous is open to question. Not a fortune, and if the pub won't bear the cost out of extra drinks sales then putting a hat around should cover it.

The link Gargoyle provided is a US example and for the matter to go to litigation I suspect there is more to the story than the report reveals.