You will need a 1.7 litre heatproof pudding basin with a lid.
Put the currants, cranberries and roughly chopped prunes into a bowl with the sherry/port (actually almost any type of spirit would do), mix together then cover with cling film and leave to steep overnight or for up to 1 week.
When the fruits have had their steeping time, put a large pot steamer with water on to boil, and butter your heatproof plastic pudding basin, remembering to grease the lid too.
In a large mixing bowl, combine all the remaining pudding ingredients.
Add the steeped fruits, scraping in every last drop of liquor with a rubber spatula, and mix to combine thoroughly.
Scrape and press the mixture into the prepared pudding basin, squish it down and put on the lid. If you're using a conventional steamer, you won't need to do this but it is recommended you wrap the basin with a layer of foil so that the basin is watertight. Put the basin either in the pan of boiling water (to come halfway up the basin) or in the top of a lidded steamer and steam for 5 hours, checking every now and again that the water hasn’t evaporated.
After 5 hours, remove carefully and, when manageable, unwrap the foil, and put the pudding in its basin somewhere out of the way in the kitchen or, if you’re lucky enough, a larder, until Christmas Day.
On the big day, rewrap the pudding (still in its basin) in foil and steam again, this time for 3 hours. Eight hours combined cooking time but it is required.
To serve, remove from the pan or steamer, take off the lid, put a plate on top, turn it upside down and give the basin a little squeeze to help unmould the pudding. Then remove the basin.
Serve with heavy thickened cream.