Rinse the berries, drain and scrape them off the panicles. Rinse lime hot, grate dry and cut in half. Squeeze one half, cut the other half into thin slices. Pour lime juice, slices, liquid sugar, white wine and currants into a punch bowl and leave to stand for about 1 hour.
Wash the lemon balm, dab dry and pluck the leaves from the stalks. Add to the punch. Just before serving, pour champagne over it. Serve in glasses and decorate with currant panicles or half lime slices
1 hour waiting time