This is just the ticket on a hot summers day! If you don't have a good amount of plums to make your own cordial try and find a good one in a store somewhere.
Plum cordial recipe:
Put chopped up plums and stones into a pot and cover with water
Boil until everything is very soft then strain.
Add in sugar - approx half the amount of liquid (or less if you'd like it more bitter)
Simmer until sugar has disolved
Ingredients for each drink:
Fizzy or still water
Fresh mint leaves
Fresh lemon thyme leaves
For each drink mix 1/3 cordial with 2/3 water (adjust depending on the strength of your cordial)
Add in the fresh herbs and ice