Thanks for the replies.
You're spot on, a longer screw may do the job as the thread at the top of the hole seems fine. I'm using the supplied fins, which have quite deep recesses where the screws make contact, meaning there's only a couple of threads to hold on.
@JBFletch, what do you mean "Utilise the multibox system"? When I pulled the side fins out I saw the obvious "Multibox" embossed inside the top of the boxes... (I know, I know, blind as a bat)
I bought the board from Kite Republic in St Kilda. Gave them a call and spoke to Cesar who was super helpful, asked for photos and will submit as a warranty claim. It sounds like they're planning to drill it out, then stick in a "plug" with a new thole.