You need

Approximately 2 liters of lemon verbena leaves stripped from the stalks

500ml lemon juice

500ml water

About 6 strips of lemon zest

250ml or more sugar

5ml tartaric acid

Place all the ingredients in a saucepan and bring to the boil, stirring occasionally to dissolve the sugar

Turn the heat off and leave the mixture, covered, to infuse at room temperature for at least 12 hours, preferably close to 24 hours.

Strain and bottle.

Refrigerate until required.

As the acidity of lemons varies, you may want to dilute the drink with more water.

Serve undiluted over ice.

Or mix with crushed ice and a splash of gin.

This will also make a satisfying warm drink sweetened with a little honey.