< the weather didn't help either as it was chucking down and visibilty wasn't great - still the moor was really atmospheric >

Sometimes I find that can be the best time to visit them!

< I believe there are 4 circles on the moor - is this true? >

Aye. Sadly the one at Bradup was recently destroyed:
http://www.themodernantiquarian.com/site/2194

But we've still got remains of the Roms Law or Grubstones circle:
http://www.themodernantiquarian.com/site/2720

The messed-up site of Backstones, which it seems only the eastern edge is part of the original (and not far from Cow&Calf):
http://www.themodernantiquarian.com/site/2733

And the very curious Horncliffe circle:
http://www.themodernantiquarian.com/site/2721

But for position and views, Apostles seems the best.

Hi Sky

The four things that I can recall up there that are stone and circular are; Twelve Apostles, Horncliffe, Grubstones and the destroyed Bradup site. I think the Horncliffe and Grubstone circles may be the remnants of kern cairns? (Paulus?)
There's also that arrangement of stones at Backstone Beck, but there doesn't seem to be a definitive answer as to what, or if, it's a megalithic beast.

These are the only ones I can think of... but Paulus may know of a few more...

Back to your original question; I usually get the the Twelve Apostles from the Cow and calf car park.
Go up the path to the left of the Cow and Calf car park. Once on the top of those rocks you should see a 'valley' in front of you, a little to the right. That's the Backstone Beck. Follow the paths that lie on the flat land to the left, along the top of the beck's little valley. Go past the Backstone enclosure walling (low dry-stone walls), and the path veers off to to the left into the moors. Walk a while, you should cross some old wooden boardwalks that make up part of the path. Head for the stone marker post on the horizon.
Once at the stone post, follow the path to the left, that'll take you there in no time!
Hope you find it!

Cheers
Dave