I found this one in The Complete book of Mixed Drinks. There are several drinks in it calling for grapefuit, lime, and something. This one intrigued me most. The book calls for Lite-n-Low which I assume is the Aussie version of Sweet'N Low. I used stevia. Here's my adaption of the recipe.
0.5 oz lime juice
2 oz grapefruit juice
1 sachet stevia
2 oz tonic water
Shake first 3 ingredients with ice. Strain in to a coupe. Top with tonic. Garnish with lime and straws.