Thanks for all replies and PMs guys. :-) I'll follow up your suggestions about contacts.
Assuming we proceed, the event will be a 1-dayer.
The landlord and landlady are close friends and music fans. They are up for it. They know what an all-day music event involves from several years of Ploughstock.
I have in mind a charity event (again, like our annual Ploughstock rock festival). At Ploughstock the bands donate their time and talent. I'd hope I could find an adequate number of folk musicians who will do the same.
I have in mind an event on a smaller scale than Ploughstock, and at lower volume. One main stage and perhaps one or two unplugged areas where people could perform or jam between acts.
I was thinking of keeping things as "acoustic" as possible maybe discouraging full drum kits and backline amplification, having just PA if possible. One thing I learnt from Ploughstock is how much time is eaten by the lugging of drums and amps...