Stab in the Back
One of the iron laws of the multiverse is that there will always be a pub within a stone's throw of a theatre, music hall, or performance venue which caters to thirsty Musicians, singers, and stage crews, to the point where the general public are unwelcome - or cannot physically get in - on performance nights. There is the Philly in Liverpool, which has traditionally hosted musicians from orchestras playing at the Philharmonia Hall next door; the Apsley Cottage is actually right next door to the Apollo in Manchester. The A.C. is hidden away at the bottom of the service road next to the Apollo just by the stage door, where the roadies unload all their gear directly onto the stage. Concealed from view by a long line of forty ton trucks, it couldn't be making itself less obvious to the general public, nor more convenient for performers and support crews at the Apollo... there is a famous story that when an American heavy rock band played the Apollo, two of its members were so enchanted to find "a genuine English country pub" so close to the theatre that they spent a couple of hours in there unwinding prior to the gig, pressing the flesh with the fans and getting to know English beers. After a while, the bass player noted the crowd was beginning to thin out, and remarked about this to the drummer.
"The gig must be starting." said the drummer, who by then was in a somewhat detached state of reality. The bassist took a long sup of his beer, and mused "Wonder how well we can hear it from here.... oh, shit...."
And then a couple of desperately searching roadies found them...
In Birmingham they've taken it further: there's a pub called the Fiddle & Bone (i.e. violin and trombone) that was bought by ex-members of the City of Birmingham Symphony Orchestra and became the de facto muso pub.
The pub that performs this function for the Opera House in Ankh-Morpork is the Stab in the Back, from which the management usually has to roust out its musicians with promises of a bonus if they'll only come back and play one more encore. (It is said they can down tools and be over there even before the applause has died down).