Concrete for a greenhouse base

Ok, this seemed like a simple one, but damned if Google / Youchoob will give me a straight answer...

I'm putting up a greenhouse in a clay soil garden, so I want a nice concrete foundation (ring, like for a house, not a slab)

What mix do I need for that base, assuming it'll be underground / water and I'll put a row of block on top for the base to screw in to?

And... if it's easier than dragging stuff through the house, can I get away with just bags of postcrete?