Argh, sorry, I see an empty distance constraint is added, but under a subtree of constraints, under control points.
A constraint isn’t a kind of control point, so this isn’t an intuitive place to look, and by the time I’d added 20 control points, the subtree (which appears above the control points) is nowhere near the “create distance” item (right at the bottom, underneath all of the points), so I didn’t see the change.
But just a UI issue.
Thanks,
Hamish