As with other commenters here, we went with unsweetened cranberry juice. We start with a pure juice, with absolutely no additives, and dilute this with chilled water at a ratio of 1:2. We also reduce the tonic water to 60ml. And serve it in a Rocks Glass. A delicious, refreshing and pretty drink.
Post Script- just checked and I used unsweetened cranberry juice- and this suited my partner and I perfectly- there is enough sweetness with the St. Germain Elderflower and Fever Tree Premium tonic. Would I use sweetened cranberry next time? No. This is perfect.
Unsweetened cranberry was just right with st germain. My apologies to Bombay but I used west winds navy strength gin which worked well with a single shot.
Perfection for a light refreshing cocktail- nothing needs changing for this masterpiece. Caveat and apologies though- no open bottle of Bombay Sapphire in the house so used the Australian gin Never Never Distilling Co. Triple Juniper gin 43% ABV. Will be made frequently in our house.