Just for the record. It turns out, that as I suspected from the quality of the packaging, the memory was not fake after all. After getting a response from Amazon (who said "talk to SanDisk"), I found that SanDisk had added a response along the lines of: "We have been reviewing the design of older cards and they appear to match yours. Can you send us some higher resolution photographs of your card...".

I couldn't get a good photo of the card because I couldn't focus close enough to get a full frame, so I scanned it instead. When I reviewed the scan and the the card a serial number had appeared. I checked the original photos and no serial number was visible, so I can only assume that the heat or UV light from the scanning head had affected the ink making it more reflective and therefore visible against the matt black card.

So I sent this number to SanDisk and the card has now been replaced.

I guess the moral of the story is: if you are sure you are right and it's not costing you anything, stick with it!