Bring the water, sugar and half of the Champagne to the boil and simmer for 2 minutes until the sugar dissolves.
Whisk in the softened gelatine sheet.
Add the remaining Champagne and immediately transfer the liquid into serving glasses or a silicone mould and refrigerate until completely set (can be overnight).
Unmould the Champagne jelly or serve directly if individually portioned and garnish with a teaspoon of the caviar.