In a pan boil the water and caster sugar together until it reaches the short thread stage.
De-seed and skin the melon. Chop it into chunks and process until smooth.
Add the cooled sugar syrup and champagne to the puréed melon, with enough water to make 350ml/11fl oz of sorbet. Churn until frozen and place into the freezer.
For the syrup, combine the vermouth, lemon zest and juice, cornflour and sugar in a pan and bring to the boil. Leave to cool.
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