To make the lavender syrup :
1 cup water
1 cup caster sugar
2 Tablespoons fresh lavender heads any variety except french lavender varieties
In a small saucepan, combine water, sugar, and lavender. Bring to a simmer over gentle heat. Remove from stove and let steep for 5-10 minutes. Strain and chill until ready to use.
To make up the drink, combine the lavender syrup above with freshly squeezed lemon juice to taste and top up with chilled sparkling mineral water; add ice and a sprig of mint.
Variation, if you can get hold of it, use elderflower and lavender syrup instead of the lemon. It is less sharp.