Tesco are doing BOGOF on 2 Litre bottles of Pepsi (any flavour) and Wilkinsons are also doing the same offer, but I don't know when Wilkinson's offer expires.
I know there's always some sort of offer on but not everyone shops around between supermarkets so may not be aware of these deals.
This seems about the best as at the moment:
ASDA are doing 2 for £2.50
Sainsburys are doing 3 for £3.00 (so when ASDA/Wilkinson expire you could go here since this offer doesn't expire till 12/08/2008)
Morrisons 69p a bottle has now expired.