Apologies if this has been covered before however 1 of the threads on my side fins has gone....clearly I wasn't using enough finesse. Is there a quick DIY fix (possibly putting a material in which hardens so that you recreate the thread?) or do you recommend I take my board in for an official repair? Cheers

Best solution is just to use a bigger screw with the same key hole, so that you do not have to use different keys. Instant repair and cheap.
Some surf shops carry them (In europe, you can get them at Gong: www.gong-galaxy.com/produit/gong-vis-de-remplacement-plug-abime-la-paire/ dunno if it is worth sending to NZ)

Otherwise, it can be repaired. The trick is to fill the hole with something that both glue heavily to the fin plug (dig grooves in it) and is hard enough to not be damaged by the screw (use microspheres and/or fiberglass bits in the resin). Then drill & tap (careful to drill at the proper angle).

See: www.gongsup.com/forum/viewtopic.php?f=32&t=3359

I have repaired mine with success... but now I just use bigger screws!

Follow what Colas said but if you can get a screw from fin boxes mounted on other brand as Starboard ones for example, the thread is slighty bigger . So you may fit it back ...be carefully while spinning and running in the new thread.