Having drawn a circle I want to use the concentric option, Modify/Circle/Concentric, or just Draw/Circle/Concentric, this option is missing! There are 9 options for drawing a circle, however, Concentric is not one of them. I can get around the problem by drawing a new circle with the same centre but a different radius but this is not what is described in the manual.
I don't know of a Circle/Concentric option and can't find anything about it in the manual.Where have you seen that? Or is it an option you are familiar with from an other CAD system?
You can edit the radius of a circle with Modify - Properties.
Or if you mean to get a circle with a certain distance around an existing one you can use Modify - Offset or Line - Parallel.
I didn't check, but possibly there was a concentric tool in 2.0, which was moved/renamed to offset in 2.1.
As already mentioned above, the Line tool Parallel or the Modify tool Offset are both usable for concentric circles.
I prefer the Parallel tool, it is more powerful and easier to use.
For the distance, you may not have recognized the tool option bar yet, which appear when the tool is active.
You find it on top of the drawing area, right beside the drawing style options.
If the tool option bar was disabled by accident, you can enable it in the Widgets/Toolbars menu again.