Coke Zero is currently £5 for 24 cans in Tesco. Just bought 4 boxes and got £18.24 refunded under Tesco Brand Guarantee making each box 44p. You must comply with their Brand Match rules to buy 10 different items. One item must be a brand (in this case the Coke) weighed items such as fruit and veg don't count. Reduced items don't count either for the 10. Tesco staff were shocked. Friend also did this for 5 boxes but the maximum discount is £20 per transaction. Not sure if other variations of Coke will work.

I'm going to try and clear this out after reading through 6+ pages and no one really explaining it.

Tesco does something called "Tesco Brand Guarantee." (Used to be called Price Promise).
Basically after checkout the till system checks to see what the shop would have cost you elsewhere.
Morrisons has (wrongly I'm assuming) been flagged as selling 24 cans of Coke Zero for ~44p, as such the till automatically reduces the coke to match this price, so instead of £5, each case costs 44p.
To qualify for Brand Match you need to buy at least 1 branded item, and at least ten items (not weighed items, like loose fruit though). So buy 4 x Coke Zero 24 packs and 9 separate other items (eg, 1x toothbrush,1xsoap,1xbread,1xmilk..etc until you have 10 separately unique/describable items in your basket/trolley).
- wixster

wixster has summed it up FAIRLY well, except it compares to 24p (not 44p) and you CAN buy loose weighted items to get to 10 different items
- aau1