Place two cups of white seedless grapes in a food processor or blender. Blend until smooth.
Pass through a colander or coarse sieve to obtain juice.
Combine grape juice with lemonade, ginger beer, and the juice of one lemon.
Place in a large jug or punch bowl with lots of ice.
Stir in the lemon slices, red and white whole seedless grapes and a handful of mint leaves. Serve chilled. Garnish each glass with a fresh sprig of mint.