Tip the sultanas, apricots, raisins and mixed peel in a bowl, and pour over the rum. Stir, cover and soak for at least 3 hrs or overnight.
Butter a 1-litre pudding basin and line the base with a small circle of baking parchment. Beat the butter, sugar and lemon zest in a large bowl using an electric whisk, until pale and fluffy. Beat in the eggs, one at a time, then stir in the flour, breadcrumbs and spices. Fold in the rehydrated fruits and peel, their soaking liquid and the apple. Spoon into the prepared basin, pressing it down firmly with the back of the spoon. Cover with a sheet of baking parchment, then a sheet of foil, and secure with kitchen string.
Put an upturned plate in the base of a large pan and sit the pudding on top, then fill the pan with water until it comes halfway up the side of the basin. Alternatively, put the pudding in the top of a lidded steamer and cover. Steam for 2 hrs 30 mins, ensuring the water doesn’t run dry. Remove the pudding from the pan and leave to cool completely.
Uncover the cooled pudding and wrap in a clean sheet of foil. Will keep in a cool, dry place for up to two weeks, or the freezer for up to three months. See tip, below, for reheating it on the day.