In a Boston glass combine a piece of pink grapefruit zest, the preserve and the sugar cube.
Soak the sugar cube in the bitters. Muddle these ingredients until a thick paste is formed.
Add two ice cubes and about half of the gin, stir until dilute, add a further two ice cubes and the rest of the gin stir until dilute.
Strain in to a double rocks glass ¾ full with ice.
Twist or flame a zest of pink grapefruit over the serving glass and pop it into the drink.