To make a larger batch for around 8 glasses - muddle approx 50 mint leaves (just the leaves, not on stems), 6oz (180ml) lime juice and 4oz (120ml) simple syrup in a jug. Add 12oz (360ml) rum and stir. Chill this mixture until needed (ideally no more than a couple hours so the mint doesn't wilt too much) Divide this base mixture between 8 glasses, each with a few pieces of ice. Top up the glasses with champagne (should use roughly the whole bottle).