Just seen this – not sure if it still works!
These are priced at £10 instore and online but are £5 in the price drop magazine (should be found in the enterance to your store or at the end of your checkouts page 17). Buy the box of Celebrations, then take your reciept to customer services. Tell them you have been overcharged as the price mag says they are £5, but you got charged £10, and the offer is valid untill 4th December, so should be £5.
So you should recieve £10 back and get to keep your chocolates. Eg, you were overcharged £5, which Tesco pay double the difference as its their fault they over charged you and it’s their policy, so you get £10 back.
Edit – should work with Cadburys Heros tubs as well