Collette, I think you should do what you want for your fast.
Fasting burns off your body fat and in the process you eliminate toxins that are stored in body fat as those fatty acids are released into the bloodstream. But there is nothing magical about it and there is simply no Science behind being a "purist" during your fast. So as far as I am concerned drinking any diet soft drinks is not going to undermine your progress. Non-caloric drinks are non-caloric drinks. They do not contribute to weight gain, which is the whole point of drinking them. :)
Some people have an insulin response to diet drinks, meaning that it makes them start craving sweets, but this is anecdotal and there is no research behind it, so just pay attention to your body and if you feel like diet soda makes you more hungry or whatever, you can drop them. Otherwise, I don't see a problem with using them for a fast.
I drank Diet Coke and other diet drinks when I was out with friends during my fast, they did not hinder my progress and helped me keep going when I had to be around a lot of socializing and food situations. :)
My feeling is anything that helps you stick to a fast is worth doing. Good luck and keep us posted on your progress!