Place the raisins in a bowl, cover with the rum and leave to soak until required. Meanwhile, blend the custard powder with 2 tablespoons of the milk. Warm the remaining milk in a saucepan. Whisk in the custard mixture until thoroughly mixed and continue to whisk over a moderate heat until the custard thickens. Remove the pan from the heat and sweeten to taste with the artificial sweetener. Cover with damp greaseproof paper to prevent a skin forming and set aside to cool. Carefully fold the whipped cream into the cooled custard. Turn into a 900ml (1 1/2 pint) shallow freezerproof container and freeze, uncovered, for 2 hours. Scrape the mixture into a blender or food processor and process well. Transfer to a bowl and stir in the cherries, raisins and rum, and nuts. Return to the container or a bombe mould, cover and freeze for at least 4 hours until solid. Remove the ice cream from the freezer and allow to stand for at least 10 minutes before serving.