Thank you for the quick and detailed reply - much appreciated! The good news is that your sample code worked exactly as expected and drew the arc. By comparing this to my code I was quickly able to identify my problem which was due to the ChartAttribute I was using to draw the arc. In your example (which works) you used:
ChartAttribute attrib11 = new ChartAttribute(Color.Blue, 1, DashStyle.Solid);
Whereas in my code, i was defining this ChartAttribute to also have a fill colour (although it was set to Transparent):
ChartAttribute attrib11 = new ChartAttribute(Color.Blue, 1, DashStyle.Solid, Color.Transparent);
This additional fill colour parameter was what was causing the extraneous lines to be drawn, presumably just because the code needed a closed polygon in order to fill it. Anyway, removing that attribute solved my problem so huge thanks !!!