If any other owners find similar this it should lead to a criminal investigation. I suspect MG can trace all hands that went through each stage of build, transportation, and delivery. It would therefore be pretty easy to identify a culprit. If it's not on other cars then it will be more difficult to identify who, where and when this might have happened. Of course there may be some design fault on your car that should be easily identified by your dealer first. If neither of these explantions has happened you need to check the local locations where this might have happened.
Do you park your car regularly in public accessible locations? Has there been any other vandalism in the area? Do you lock the car or rely on the auto locking when leaving the car?
If you park on a drive can you monitor the car by a camera? Even if you park in a garage I would still use a security camera nowadays.
I hope this is resolved soon so please keep us updated and everyone check your seat belts. I'm going to do that right now.