Heat the water, sugar and lavender in a small pot of high heat. Bring to a low boil, then cover and remove from heat. Allow the simple syrup to steep for 20 minutes. Pour through a sieve to remove the lavender. Place in the fridge until ready to use.
For each cocktail: Fill a glass half way with ice. Pour the simple syrup, lemon juice and gin over ice. Stir, then top with club soda.
For a martini-style Tom Collins: Fill a cocktail shaker with ice. Add the simple syrup, lemon and gin. Cover and shake for 30-60 seconds, until frothy. Pour into a martini glass and add 1-2 ounces of club soda.