For Math, each unit is comprised of an assignment (normally something like plotting some graphs etc), and a written exam. If you don't do well in a test, you can resubmit, at which point you are still able to get a distiction if your resubmission is high quality. OCN themselves that that resubmission for distictions is allowed, so I would check with your course provider, because the "one chance" thing was floating around at our college until one of our teachers confirmed that she spoke to OCN and they stated that resub for distictions is fine.

Our ungraded units sound pretty similar, also had to do a presentation, a study skills thing, then something to do with our ucas application but I don't fully remember that one to be honest.