It hasn't recently been said but the guidance on incidental is pretty unhelpfully written.

Surely what it is trying to say is on the lines:

If you, a member of Joe Public, go to a pub where there is no session, you drink and maybe indulge in other activities typical to a pub. If one night there is a session or some other entertainment and it prevents you from indulging in your normal activities in the pub, whether because it is so loud that you can't hear yourself talk or in some other way, then it is not exempt. If you can indulge in all the activities you normally do then it is exempt (even if you prefer to listen to the music instead of doing what you normally do). However if you stop drinking the pub is unlikely to have sessions very often - because whatever you do in a pub the most important thing to the landlord is buying his booze.

What I have written is easy to understand but it is not the language of the law.

It is not often appreciated that the statutory guidance is just as much law as the act itself. It has equal weight in the legal process because it has been published as a statuory instrument, which is the most common method of making law these days. Acts of Parliament almost invaraiably just lay down the context in which statutory instruments fill in the detail. SIs can even reverse the law as passed in the original Act of Parliament.