Hi Tom
The shark fin should be tight.
It is quite easy to do:
Remove the rear interior light (prize out the plastic bit and once removed the light assembly slides and drops out).
If you look through the lining you can see two nuts (10mm spanner I think).
Tighten the nuts and job done, it will take you about 15 minutes. Hope that helps.
Gary
It should be tight, but I have no idea how you would tighten it. Probably best to ask a main dealer if it something you are able to do without disturbing the head lining.