Works out at just 22p per can. (Thanks to 'Common Sense' for correcting my non use of calc )
Went into my local waitrose yesterday and spotted the deal. Checked and its also online
Comes up as
Special Offers - Diet Coke 6 Pack
DIET COKE 2 FOR 1 Offer Ends: 24/02/2009
Also some deals on various sized bottles. Link not the best but its either the main web page or the details of the deal unless someone else can figure it out.