Put the tea in a heatproof jug and pour over 1 litre just-boiled water. Leave to steep for 10 minutes, then strain the tea leaves and discard. Leave to cool to room temperature, mix with the elderflower cordial, to taste, and chill in the fridge.
Add a few ice cubes, mint leaves and strips of lemon zest to each glass or jar. Pour over the tea, garnish with another sprig of mint and serve.