Thanks to both of you, but, unfortunately, none of your suggestions have fixed the problem.
However, I have managed to find a solution, and it's a doddle, really!
All one needs to do is convert the workflow into an application, by selecting File>Convert to and choosing "Application". One must be careful, though, to make sure it is saved as an application; specifically, once the workflow is converted into an app, hit command+save, and in the popped up window, set File Format to "application" (note that it is set to "workflow" by default).
This is simple to do, of course, though it has become a small bit of a niggle, hasn't it? I do not quite understand why Mountain Lion impels us to make a (not exactly trivial) extra step where Lion didn't. Must be something about the low pressure up there...