Check out the council H&S area on their website, or google it. 'Most' councils have such information available to show what they would expect.

What is actually required is how you will be managing the risks produced or affected by the concert.

There is no legal requirement for a formal risk assessment (but if that is what you want to do fine) but councils love to ask for one as it's all they know.

The council has it's own legal duty to provide you with information - their fire arrangements, access & emergency egress, any limitations as to use of certain items, noise limitations etc. Without this you cannot do your bit, and it also helps if you have an officer contact with whom you can liaise, so submit 'simple' first, as it may be enough. The run these (and far riskier) events all the time.

Concentrate on the 'added impact' risks - safety of the public, how you will deal with emergency evacuation including for those needing assistance, noise, food poisoning, that kind of thing. The council is not interested in the safety of individuals involved (the 'contractors' if you like) such as manual handling, PPE etc as these are not licencing issues. That is not to say ignore, but it's only significant to the participants.

Put together a plan to include numbers, (profile of attendees is good), stewarding, first aid, communication in event of an emergency, people with disabilities, fire protection (no pyrotechnics is a common one). Bullet point the key rules, and have a list of organisers, their roles and contact numbers. Your plan should include the safety briefing for stewards, who need to be made aware of the building layout, equipment and alarms.