Tesco are currently selling 45g M&M's or 35g Revels for 49p each or 2 bags for 80 pence.
These "Bag a night In" branded packets come with the blockbuster codes inside and you will need 3 codes for a film rental (80p + 49p = 1.29p).
Create an account as per the instructions on packate ( via bagabignightin.com)
Enter the 3 codes inside the packet.
Print out voucher and take to Blockbuster.
You can of course eat the chocolate at any time!
Rental not restricted to any specific day of the week.