Please have a look at the attached screenshot.
You'll see I have 2 test fixtures but the JC Unit test Window lists 3.
Now, I know what's happened, I just don't know why or how to correct it.
The "BaseDigestFixture" class was renamed to that from "DigestFixture". However, it was a rename and the original file no longer exists neither in the solution or in the file system. Restarting VS hasn't cured the problem so it doesn't appear to be some sore of temp caching issue.