Wash, clean and slice the strawberries. Halve 1 lime and squeeze it. Mix strawberries, lime juice and elderflower syrup in a bowl. Chill for about 30 minutes.
Wash 2 limes hot, dab dry and cut into slices. Wash mint and shake dry. Pluck the leaves from the stalks. Put the strawberry mixture, lime slices and mint into a punch bowl. Add mineral water and champagne.