Combine the mint, thyme and eucalyptus leaves in a blender and whiz. Add the aniseed and cloves to the blender and whiz again. Place in a dark bottle with the vodka, sorbitol, if using, and lemon and leave for 10 days to 1 month to macerate. Strain through muslin. Stir in the glycerine, and then pour into a 50-milliliter spray bottle. Spray 1 milliliter into the mouth when needed. This contains alcohol, so be careful not to overuse, especially if driving. Do not use if pregnant. Keeps for up to 1 year.