EDIT: June 4th
As you can see from the comments, the card turned up at Tesco too. Their response was just as fast, and the card was removed within an hour of my speaking to a manager. Thank you Tesco!!!
EDIT: June 5th
Received a phone call from Sainsbury's HQ confirming that the card had since been removed from every single store and the matter taken up with the manufacturer. Apologies for offence caused. :-)