This really is a doddle.
All you do is first sift the flour, baking powder and mixed spice into a large bowl, then add the eggs and softened butter. After that, spoon the treacle into a glass measuring jug, add the stout and rum and mix together. Next, pour this mixture into a bowl (it will look lumpy at this stage but don't worry) and, using an electric hand-whisk, whisk everything thoroughly together. Now add the apple, nuts, currants, mincemeat and finally the grated orange and lemon zest.
Then, using a large spoon, mix everything together thoroughly and pile it into the pudding basin. Now take a double sheet of foil, measuring 12 inches (30cm) square. Make a pleat in the centre, then place it on top of the basin and twist it all round. Next, place the basin in a shallow roasting tin.
Pop it in the oven, the pour 2 inches (5cm) of boiling water from the kettle into the roasting tin and leave the pudding to cook for 2 hours 15 minutes, or until it is springy and firm to the touch in the centre. You may need to top up with water from the kettle halfway through the cooking time.
Then remove it from the oven and leave to cool.
On Christmas Day, when you have taken the turkey out of the oven, reduce the temperature to 180C, gas mark 4 and place the Christmas pudding back in the roasting tin with the same amount of boiling water as before and reheat it in the oven for an hour.
Or you can reheat it in an 800W microwave for 6 minutes (remembering to remove the foil and replace it with clingfilm that you have pierced with a fork a couple of times).
For the sauce: all you do is warm the custard and mascarpone, whisk them together in a saucepan then add the rum and keep tasting and adding till you have enough.
If you want to flame the pudding as it goes on to the table, place it on a large serving plate, then warm 2 tablespoons of rum in a ladle held over a gentle heat and test it with your finger to make sure it's really warm before setting light to it.
Carry the ladle with the pudding to the table, slowly pouring the flames over the pudding.
Or you can simply dust with icing sugar.
NOTE: The best way to warm up the leftover pud on Boxing Day is to wrap it in foil and reheat in a lowish oven for 15-20 minutes.
Pre-heat the oven to gas mark 4, 350°F (180°C).
You will also need a 2 pint (1.2 litre) pudding basin, lightly buttered.