For the pudding, place the raisins and sultanas into a bowl with the rum and 200ml/7fl oz water and mix to combine. Set aside for at least four hours, preferably overnight.
Place the remaining ingredients into a bowl and mix to combine. Add the soaked fruit and any residual liquid and mix to a make a soft dough.
Lay a large piece of aluminium foil onto a work surface then spoon the dough onto it, forming it into a log shape. Roll the dough up in the foil, twisting the ends to secure. Wrap the roll in a piece of muslin or, as was traditional, in a shirt sleeve, and tie the ends with kitchen string.
Place the roll onto a heatproof plate in a large pan, then pour over enough water to cover the plate. Cover the pan with a lid and place over a medium heat on the hob. Bring to a boil, then reduce the heat to a gentle simmer and cook for two hours or until cooked through. Top up the water as necessary as the pudding cooks.
Remove from the pan and leave to rest for five minutes then remove the muslin – or shirt sleeve - and the foil.
Meanwhile, make the custard. Place the egg yolks and sugar in a bowl and whisk until thick and pale.
Place the milk and cream into a saucepan and bring to the boil. Pour onto the egg mixture, whisk well then return the mixture to the pan and heat slowly, until the custard has thickened enough to coat the back of the spoon.
To serve, slice the pudding into thick slices and ladle the custard over the top.