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probably curse the day you started it
You're probably right, but I can't think of a simpler idea that is going to satisfy the itch now. I was going to put the whole thing on a concrete slab, overkill do you think? I think you advised that for large sandstone slabs, but maybe the smaller setts are more forgiving of ground heave. The soil is clay 100mm or more down so probably not the most stable. I fully expect to have to chop a lot of blocks in to odd sizes.
Polygon-
I tend to advise concrete bases for most things, once they are down they stay there, particularly over clay. A couple of mm of movement and your pretty pattern will be less so. Don't forget to give it a degree or so of fall to clear water.
Please photograph your build stage-by-stage and append your psychiatry report.
ColinTheBald
Perfectly doable, but you'll probably curse the day you started it. Your method is just fine, but remember to set your outer ring on mortar to retain the rest and allow for approx 2" of sand within the outer ring plus the depth of the setts. You'll need a Wacka plate and a block splitter/saw/angle grinder with diamond blade to cut down the setts that inevitably refuse to fit. No matter how hard you try, the calculations of number of rings of setts plus kiln dried sand between them won't exactly fit the outer ring. Here's one I did of various reclaimed bricks of different shapes and sizes we dug out of the garden whilst excavating a new formal patio. It was an absolute sod, but less complex than your design. Enjoy!