Unless you are prepared to go to court, there is is little point in forming a legal argument (under licensing law), for the officers will now just provide a different one, they already have. The only place this would then ever be settled is in court (and by then the new legislation would most probably be in effect).

The best chance, out of court is to inform and pressure the elected council members into instructing their officers to find a solution, where they can carry out their statutory duty but to enable sessions etc.

The new law is unlikely to be in when the Sheffield carols start up so a solution really must be found. And I assume that the Palm Tree session is still being prevented? There is the Human Rights Act that the Palm Tree participants could use.......