Pour the strawberries and alcohol in a large pan or bowl and using a masher, extract all of the strawberry juice from the strawberries (or alternatively, you can use a hand blender here). Use a very fine sieve to strain and remove all of the seeds and larger pieces of fruit.
In a separate pan, dissolve the sugar in water over a low heat, then gently simmer for 5-10 minutes, or until a thick syrup forms. Set aside to cool.
In small stages, slowly add the sugar syrup to the strawberry mixture, tasting and checking for the correct sweetness as you go. Pour into a bottle and seal until needed.
When ready to serve, squeeze the lemon juice into a bowl and turn each champagne flute upside down so that the rim is lightly coated in the juice. In a separate bowl, sprinkle in some caster sugar and repeat the same method so that that glass rim has a coating of sugar. Pour about 35ml of cassis into the bottom of each champagne flute and top with champagne. Decorate with an extra fresh Jubilee strawberry and mint leaves. Enjoy!