The councils have endless discretion. to say they haven't is pure horlicks. Simple

I don't know how it is in the UK, but I've been involved in local government here in the US, and on many things involving public meetings and events there are statutory requirements that advanced notice must be given and local government has no discretion to overrule these.

These laws were written for the protection of the public so people could make appropriate plans or arrangements to deal with noise, congestion, etc, or in some cases so there would be time for a comment or response before whatever it was actually took place.   So I wouldn't be surprised at all if the Cornwall Council's hands were tied on this matter.