As some have perhaps implied, you don't do ONE risk assessment for a whole festival, but sub-divide it into different events and venues within your total festival, and do a risk assessment for EACH, as risks may be different in each place, and in the same venue under different conditions of use.
Our Festival director has developed a couple of simple forms which have to be filled in and returned to her before the start of each event, using a scoring system, and addressing any problems elicited in advance of the event. Each event has a nominated person in charge so that it does not all fall on one head, but yet is done in a standardised way: that person will both do the risk assessment and steward the event.
And as someone else has said, sometimes it does mean stating the obvious, so that if there should be any claim against us, we can show (in writing) that we have thought about all the hazards and done our best to minimise them.