If we look at the history of the 'incidental' exemption - it was originally only for 'incidental recorded music' such as that provided in lifts and background (juke-box) type things.

Now it would be a strange person indeed who seriously tried to maintain that advertising the fact that you had music in your lift would turn this into public (or now regulated) entertainment.

Now that the exemption is extended to all 'incidental' music - I would suggest that just advertising a regular session was to take place would not turn this 'incidental' live music into public (or now regulated) entertainment.