The word "entertainers" appears on the window notice and in the advertisment. A darts match of only 2 would be pretty feeble and caselaw says that 2 means 2, not 2 + 2 +2 etc.

Good point. So a darts tournament of two sides of 4 (or whatever they have" would be illegal if the darts players are regarded as "entertainers". If they are not neither would sessions musicians I would have thought. As I say, ignorance by the Brewery and the Landlord. I have seen many other such notices just specifying "regulated entertainment as specified in the application". The advert and the window notice does not detail "what is in the application".   

Incidentally the pub with the "maximum 2 entertainers" is not in a residential area but near a run down wholesale fruit market.