Lavender simple syrup: Place all ingredients into a small pot and heat gently until sugar has dissolved. Simmer gently for 10 minutes. Leave to cool. Strain liquid. (Will last refrigerated for up to a week).
For the cocktail: Add gin, lavender simple syrup and lemon juice to an ice-filled cocktail maker and shake well. Place a lavender flower into a champagne flute, pour in lavender syrup and top with bubbles.