Ok, so the deal is this:
Sprite have currently got an offer on which gives you 50p off your mobile phone bill (Monthly Contract) or 50p credit (PAYG), all you have to do is to text the code printed on the inside label to 85888.
According to the terms and conditions, you can do this up to 1 time per day, with a maximum of 8 times per mobile number. So that is a saving of £4.
Sainsbury's currently have the 500ml bottles of sprite on 2 for £1.30, so if you were to buy 8 bottles that would set you back £5.20.
Offsetting the saving of £4 on your phone bill/credit against the purchased sprite, means that you get the 8 bottles for a total of £1.20!