No one's mentioned the Licensing Act yet. The pub's licence may not extend to an event of this nature so you may need a temporary events licence.

You'll also need to consider health and safety (do a risk assessment), also disabled access. Make sure its understood what areas are the pub's responsibility and what are the festival's - it's easy to assume someone else is dealing with a particular issue. Get the Red Cross or St John's Ambulance to supply first aiders.

Make sure there's reserved parking for artists, and access so they can bring their vehicles as close as possible to the stage, for loading and unloading gear. They'll also want to know their valuable gear will be safe, so make sure there's always someone in attendance between the soundchecks and the performance.