For the jelly, soak the gelatine leaves in a shallow dish of cold water to soften. Meanwhile, gently heat the tonic water in a pan over a low heat. Drain the gelatine leaves, squeeze out any excess water and add to the pan with the tonic water. Stir until dissolved, then add the gin. Taste the mixture - if the gin flavour is too harsh, add more tonic water.
Pour the gin and tonic mixture into a shallow container and set aside to cool. Chill in the fridge for at least 4 hours, or until set.
For the mousse, heat the milk with the juniper berries in a heavy-based pan. When the milk comes to the simmer, remove from the heat and blend the berries and milk together with a stick blender. Pass the mixture through a fine sieve into a clean, heavy-based pan.
Place the pan back over a low heat, then add the white chocolate, stirring occasionally, until the chocolate is melted and the mixture is smooth and creamy. Pour the mixture into a bowl and set it over another bowl filled with ice water to cool the mixture down.
Whisk the cream in a bowl until soft peaks form when the whisk is removed. Gently fold into the cooled chocolate mixture, then pour into a shallow dish or container and chill in the fridge until ready to serve.
For the sorbet, place the sugar and boiling water in a non-reactive pan and heat until the sugar has dissolved. Stir in the lemon juice and glucose, then pour into an ice cream maker or sorbetier and churn according to the manufacturer's instructions. Transfer the sorbet into a freezer-safe container and place into the freezer until ready to serve. Remove the sorbet from the freezer 10 minutes before serving, to allow it to soften slightly.
Preheat the oven to 180C/360F/gas 4. Grease a baking sheet with butter.
For the ginger snaps, place the butter, sugar and golden syrup into a pan and melt over low heat, stirring occasionally.
Sift the flour and ground ginger together into a bowl, then mix in the lemon zest. Mix the butter and syrup mixture together with the dry ingredients until you have a soft dough.
Place spoonfuls of the ginger snap mixture onto the baking sheet, keeping them well spaced apart. Bake for 6-8 minutes, or until golden-brown but not too crisp. Remove from the oven and immediately shape into curls (a thin rolling pin is good for this). Leave the curls to cool and harden on a wire rack.
To serve, cut the gin and tonic jelly into cubes and place a small pile at one end of the plate. Using 2 warmed spoons, shape a quenelle of white chocolate mousse and place on each plate. Scoop the lemon sorbet into the chilled martini glasses and stick a ginger snap into the sorbet. Serve immediately.
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