With the 2 for £6 deal, you're getting 12 1.5L bottles of Evian i.e. they're just 50p/1.5L bottle equivalent.
For comparison, the 6 x 1.5L sets are:
- Asda: 70p/1.5L
- Tesco: 63p/1.5L
- Morrisons: 70p/1.5L
- Sainsburys: 53p/1.5L
Probably not worth paying the delivery charges just for this product as they'll eat up any saving, but if you're going to do a shop at Ocado anyway this is the best price for Evian right this minute from the major supermarkets.