I read about this and kind of forgot about it, but today I got to try it and it works.
At the service desk in Tesco is a big notice. At the bottom it says 'if we charge you more than the marked price we will refund you in full and you keep the item.
If the shelf label OR price sticker are less than the till you can get it free. (ie if they are different) People have been using this to get free TV's...joints of meat... you name it.
I can't be bothered to look for such things. This morning I went in to get a sandwich and was tempted by the buy 6 get 6 free sign on the yoghurts. No discount apart from -20p came up on the bill. Went to desk said I want a refund and keep the goods as I have been overcharged as per the sign. (read you have to ask or they just give you a correction amount back).
No argument - money back and I didn't get the 20p taken off... so they paid me 20p to take away 12 yoghurts
Happy hunting.