Put all the ingredients, except the lemon twists and raspberries, in a cocktail shaker. Shake until well chilled, then strain into 2 Martini glasses. Add a lemon twist to each glass and raspberries (if using) to decorate.
You can buy ready-made sugar syrup in store, or make your own by heating equal parts of caster sugar and water together until dissolved; cool. This will keep in the fridge indefinitely in a sterilised bottle (steralise your bottles for 5 minutes in a medium to hot oven (190°C, fan 170°C, gas 5)