To make plum and lemon syrup, combine ingredients (except lemon juice) in a small saucepan and bring to a simmer, stirring gently until sugar is dissolved. Simmer for another ten minutes. Steep covered, overnight up to 24hrs. Strain and remove all stones and pulp. Bring back to a simmer, add lemon juice and a pinch of salt. Adjust to taste.

Add gin and plum & lemon syrup to a mixing glass with ice, stir down til just at temperature. Pour into a chilled highball, wine glass or large coupe (min 240ml capacity). Top with dry ginger ale and serve with a orange half-slice on the rim.