Hi Paul,
Rivington is pretty good, however it does get a little busy. The better place is about 10 mins drive further into the Moors near Darwin, a place called Tockholes. The South end of the plantation is usually pretty quiet any time of year and has some good foraging and wild life too.
There isn't much flat land about Rivi and what areas there are, are all within the Lever park area, and the reality is there should be no camping. However I always see small groups pitching up in the evenings and they don't seem to get moved on.
Still roughly in the same area as Rivi are Lead mines Clough, Healy Nab and Anglezarke Reservoir. they all have potential areas for a bit of overnighting but I have yet to stop over at any of them. I have spent a quite a few hours in day camps at them.
I'm in the middle of trying to get some proper permissions to use certain areas from United Utilities, but have had no luck as of yet.
Mike