Asda have the 2L bottles of coke on offer at the moment 2 for £2.
But coca cola have their own promotion at the moment too, as long as you pick up the promotional bottle..
The offer is, buy a 2L bottle of coca cola, get the 12 digit code of the label, enter it on the website, and get a voucher (printed) to redeem a free 2L bottle of 'coke zero'.
So if you buy 2 bottles of coke for £2, you will get 2 codes, resulting in 2 free 2L bottles!
Info on the coca cola website