I made a pact with myself that as long as I drink plenty of water, I am allowed no sugar "treats" to drink. I tend to use diet fizzy drinks as a pick me up if I'm feeling particularly low, much in the same way I would have turned to a bar of chocolate on a bad day before I was diagnosed.
So although consuming these chemical laden fizzy delights is not the best course of action, they certainly won't impact blood sugar.
My repertoire includes: diet vimto (both fizzy and cordial), sainsbury's diet dandelion and burdock cans, diet irn bru, diet coke, coke zero, diet pepsi, pepsi max, robinsons no added sugar (always have a variety of bottles of cordial available, and I drink a pint of weak cordial every night with dinner), Herbal Teas: peppermint, buttermint (tastes like murray mints), chammomile tea, and anything else herbal. I don't like fruit teas very much.