What is "Common Purpose"?

Googling this term, one gets (a) lots of stuff by Common Purpose and/or a Julia Middleton, (b) lots of favourable stuff about either or both, and (c) a handful of snarling "stop-them" sites. I shouldn't bother asking any more, except it seems that this organisation does have connexions with many influential people and organizations.

What is the "Common Purpose" of the eponymous outfit? Who exactly is this Julia Middleton, and why has she become so apparently prominent? Is it a McKinsey-type thing, or a McCarthy-type thing? Can anyone who isn't obviously a nutter tell me, in cool and rational terms?