I was given the good advice my first time at Sidmouth (in Steve Heap organising days) to buy a Season Ticket then you can dip into anything that needs paying for (except for some stuff now at the Ham, a large sweaty marquee venue though host to big names).
I am no longer a Seasoned Ticket holder but it was good advice. I don't use the festival campsite either. There are half a dozen permanent sites East of Sidmouth which are served all day and after midnight during festival week by a dedicated bus.
I only B&B'd once. Did find a place in town but it was a struggle, booking in January.