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Concrete!? Interesting. The videos I was watching last night suggested 4"+ of stone dust then a 1" dry mortar bed was best. I'll be building a retaining wall around the perimeter with a 400mm foundation and 2-3 courses of concrete blocks on their sides. That should help keep the whole lot in place. It'll sit below ground and separate the beds and the patio. I'll set a ring of coping bricks on top as a border.
The Marshalls stuff is 'calibrated' which should helpfully help get them even. I'll have to install some kind of drainage channel along the side of the house I imagine
Polygon-
I've used Marshalls, don't trust the calibration, although it is generally quite good. Don't try to lay a bed of mortar all at once, do one or two stones at a time.
The cardinal sin when laying random sandstone is getting four corners together and forming a cross. Also, long unbroken lines are to be avoided. That bloke manages both in a small area, I would sack him. His method of laying guarantees voids and therefore weak spots in the mortar.
I always use a concrete base, yes it costs more, but it goes nowhere, particularly on clay soils which expand and contract considerably.
I am sure you would do this anyway, but lay out all of the sandstone circle, including joints, to set the exact position of your wall. You can extend the concrete circle and use this as wall foundations as well.
ColinTheBald
OK, so I'll try to give a quick summary. Neither is as easy to lay as slabs, which are generally laid on studs of mortar.
Blocks are laid on a 2" bed of carefully screeded sand, then compressed with a wacka plate, which is used again to rattle in kiln dried sand to bond them together. It's important to get the base even or the whole lot will be bumpy. The outer ring of blocks has to be set in mortar or everything will spread out. You also have to allow for a few mm of compression for the sand or the centre will whack down lower than the perimeter and hold water.
Sandstone should be hard wearing if laid correctly, on a continuous bed of 1"/25mm of mortar. The varying thicknesses of the sandstone makes an even surface difficult to achieve. Then all the joints must be pointed.
Either way, I would recommend a 4" depth of concrete beneath or the whole lot will move.
Enjoy!