So back to the shop with it for a refund, where they looked rather puzzled for a bit before remembering some email from head office - apparently there's a few fake Amiibos circulating and they think this is one of them. It did look a bit tatty with some scuff marks on the paint but I'd just assumed that was wear and tear.
The weird thing is that it did have a NFC tag in it, just not one with the necessary Amiibo coding. I mean, if someone's going to go to the trouble of making a fake Amiibo then why would they bother putting a non-Amiibo NFC tag in it? What's the point?