The more I think about it, the smaller group seems the way to go. I believe Griff's 'group' includes family members and long standing friends. Even if four or 5 folk got together, self managed running costs wouldn't be excessive surely. I reckon that I spend about 2.5k annually for the equivalent of around 4 weeks hire. That would be 10 - 12k which would surely go a long way to covering operational costs. Plus a smaller group would have the potential to be more flexible and even come to an arrangement where members could be guaranteed 'cherished' holiday times or school holidays for example.
I can think of one Broads loving couple who I could happily share with but have the advantage of knowing them well!!