To make a mincemeat syrup, put 100ml water, the golden caster sugar and mincemeat in a pan and bring to the boil. Cool and strain though muslin into a sieve. Separately, mix the caster sugar and mixed spice, then dampen the rims of 4 Martini glasses and dip in the sugar and spice mixture.
Muddle the mincemeat in a cocktail shaker (you may need to make in 2 batches), then add the gin, sweet red vermouth, dark rum, 80ml mincemeat syrup and ice. Shake well and strain into your prepared glass.