Thanks everyone for your help and an interesting debate. I have bought myself a bottle of Coke Zero and really enjoyed a small glass with my tea, a nice treat after days of water. I will see how it goes.
It's not great for you whether it spikes blood or not. The carbonation can leach calcium from bones and sweeteners have all sorts of unpleasant side effects. My biggest worry though is that it continues the need for sweet flavours when if you can give up diet favoured drinks you will change your tastebuds much more towards the healthy diet you can enjoy.
Does that diet work ok for you? Do you find you have to eat every couple of hours? I'm still new to RH and as everyone is different it's nice to hear how others control theirs. Hope you don't think I'm prying