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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-
Where do you work by the way? Fancy coming to Cardiff? Sounds like you'll do a considerably better job than me!
Good shout on the concrete actually, it won't be that much more than what I need to lay for the reatining wall, and I can probably do away with that all together. Considering I've dug out about 100mm -150mm lower than where I want the top of the patio to be, how would you go about making a circular former for the base?
I always use a concrete base, yes it costs more, but it goes nowhere, particularly on clay soils which expand and contract considerably.
Polygon
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