Tried the method without dicing into segments and it nearly worked! But I only used a small bit of stone (not willing to nail a big flag).
Using the "cake cutting" technique, and about 10 segments, scoring 1/3rd the way through each side, used a cold chisel and club hammer in the scored grooves ... worked a treat.
The 4 1/2 inch grinder had a diamond blade on and actually made the curvature quite well, in the end this was a lot easier than I thought and the result is fine for my purposes (when pointed in).
See
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for result.
Cheers,
Alex.