Thats a sore one, I would have thought that as the STA has admitted that its their fault, it would be down to them to rectify it. No fault could be levelled at the weavers door I would imagine. Maybe the STA could register the wrong tartan themselves under a different name and sell it on?
I hope you get this sorted out okay, I wouldn't like to think you end up with the wrong tartan, good luck....