Finishing off the edges with half round logs was the objective. These were nailed into the edge of the decking, but with a further noggin/spacer nailed on at about half height, and sawn to fit up against the joist under the overlap of the decking planks.
The corner was cut to a curve. After much deliberation and erroneous application of "O" Level geometry it was finally done by eye freehand , and cut with the jigsaw. Note the plastered thumb in the pic.