As video cameras are widely available and easy to use, it seems sensible to at least consider using them to assist you in the management of your event. I have listed below some of the opportunities for using a video camera and the potential benefits that can be gained.
At the start and finish of your event, check the entire site, inside and out. Wherever possible, do so in the company of the site owner, using the site owner and their voice on the tape. This proves that the record was taken in the presence of the site owner or his representative, and allows you to take a verified record of the state of grass, gates, fences and hedges. The pictures will later help to prove if a fence the landowner claims was broken during your event was already broken or if it was caused by somebody at your event.
If there had been an accident or incident, as soon as possible I would try to take a series of pictures of the scene from all angles. An insurer may not require the pictures, but they will certainly be a useful aid to ensure that similar circumstances are not allowed to occur during the rest of the event, or in subsequent events.