This is the Bloke's department - far too much precision involved for ditzy old me. The slabs are a sort of mellow limestone left over from a landscaping project on a far posher house than ours - hubby was the carpenter so he came home with a pile of this really rather lovely, though somewhat brittle paving. Since it did crack so easily I was fretting about how to use it - heavy-duty pathways and the like were entirely out of the question - but it turns out it's a shoo-in for crazy paving.
One sticking point, if you'll excuse the pun, was the cement: I simply cannot figure out how you're supposed to lay paving around a pond to hide the edges of the liner without getting cement in the water. This is obviously A Bad Thing as it poisons fish (and no doubt sundry other wriggly things) but absolutely impossible to avoid. In the end we stopped worrying about it too much and simply re-cleaned the pond at the end of the operation.
The finished product: after clearing up those left-over slabs it'll be the tidiest thing in our garden. Next step is the bog garden and a lot of plant shopping. Now that is my department.