Preheat the oven to 180C/350F/Gas Mark 4 and grease and line a pudding basin.
Place the dates in a bowl and pour over the milk. Stir and soak for 20 minutes.
Place the butter, sugar and eggs in a separate bowl. Sift over the flour, cinnamon and ginger and beat with an electric mixer for two minutes. Add the soaked dates and walnuts and stir well.
Spoon the mixture into the pudding basin. Place the basin in a roasting tin and pour in enough boiling water to come just 1cm up the sides of the basin.
Loosely cover the whole thing with kitchen foil and bake in the oven for an hour.
Meanwhile, to make the sauce, drain the canned fruit, reserving 150ml of the syrup. Finely chop the mandarin pieces. Blend the arrowroot with a little of the juice in a small saucepan, then stir in the rest of the juice. Bring to the boil and simmer for one minute. Stir in the marmalade and simmer for two more minutes, stirring occasionally.
Remove the foil from the roasting tin and turn the pudding out onto a serving plate. Spoon a little of the hot sauce over the top, then pour the rest into a jug and serve with slices of pudding.