Oxjam doesn't usually recommend holding an outdoor event. They carry a high risk (especially in October) and take a lot of organisation. The best formula for a successful event is an existing, inside, licensed venue which has the equipment that you and your bands will need.
However if you do feel experienced enough to organise an event anywhere live music is not usually heard - the most important thing that you need to consider is a Temporary Events Notice (TEN) These are usually available from your local council. Find out more from www.culture.gov.uk. You will also need to ensure your event is covered by Public Liability Insurance.
It sounds obvious, but don't forget - sufficient waterproofing for all your electrical gear is essential. Don't stand anything electrical or valuable directly on the ground to avoid puddles forming unexpectedly and causing damage. Make sure everything is well tied down in case of high winds. Please make sure you complete a risk assessment for your event.