I never figured it out and rather than buying replacement clips (both were broken when I bought the car) I just used some M5 nuts and bolts, heated the plastic hole then wedge the head of the bolt in, put a single nut to stop it falling through under the trim. Then drill a small hole in each of the clips that can then slide over the bolt, the thread holds it in place but you can easily take them off to remove parcel shelf etc.
It's worked perfectly and doesnt stand out too much.