I contacted Samyang Australia and they suggested sending it back to Korea for fixing at $200. To be honest, the money wasn't that big a deal, but being on the road, the loss of the lens was!
So I have spent a few hours stripping the lens down, only to find that the fix was incredibly easy
Nothing more than removing the rubber grip, loosening three grub screws and rotating a silver collar until the correct focus is achieved. Job done...
(An almost identical method is used on the 14mm I noticed on google)