That's often the problem. What's advertised as a session (I'm not necessarily talking about Middlewich of which I know nothing) quite often turns out to be a band performance in all but name. A session isn't just playing along with a lead musician; a good session is like a conversation, involving give and take, and does not have a predetermined agenda.
Sound like the problem is advertising and making what's 'on offer' clear. Session obviously has too many meanings and should erhaps be clarified?

I don't think I've ever been to a session where I haven't paid somewhere along the line (ticket price or popping money in a bucket on way in).

If your looking particularly for sessions....then maybe your looking in the wrong place. If you investigate carefully there are events that have 'sessions' and no 'big name bands' playing on a stage, but they might not be called 'Folk Festivals'.

I have to say when I go to festivals I always pay the ticket fee....then only see a handful of acts coz I'm too busy in the session (and have to admit when I've found the official sessions to fast or hard for me sat down in a quiet spot, started playing and was soon joined by other beginners) or going to the workshops.