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.
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.