First, make your muslin food-safe by soaking it overnight in cold water, then boiling it for 20 minutes. Rinse it in cold water and hang it in a clean room to dry.
Lightly flour the blades of a kitchen grater and grate the solid suet into a bowl.
Fill a large pot 3/4 of the way full of water, cover it, and set it on the stove to boil.
Meanwhile, in a large bowl, combine flour, sugar, nutmeg, ginger, cloves, and salt.
Rub the grated suet into the dry ingredients until no pieces larger than a pea remain.
Add mashed potato and mix until thoroughly combined.
Add raisins and currants and mix until evenly distributed.
In another bowl, beat the eggs until smooth.
Stir the milk into the beaten eggs.
Add the milk and egg mixture to the batter and mix thoroughly.
Submerge your prepared muslin in the pot of boiling water. Wearing heavy rubber gloves, remove the cloth from the water and wring out the excess moisture.
Lay the cloth flat on a clean countertop and liberally sprinkle the center (where you will place the plum pudding) with flour. Rub the flour across the cloth, ensuring that a circle of at least 16 inches in diameter is coated with flour, and that the flour layer is slightly thicker at the center.
Place the batter on the floured cloth. Gather the cloth up around the mixture and, using the cooking twine, tightly cinch the cloth as close to the mixture as possible. Knot the corners together for a more secure seal.
Lower the cloth into the boiling water and cover the pot. Boil the pudding for four hours, replenishing the water as necessary.
Lift the cloth out of the water and place it in a colander. Cut the string, open the cloth, and turn the dessert out onto a plate to cool. Let sit for at least 20 minutes before serving.
To store your pudding for later use, allow it to cool to room temperature, wrap it in plastic wrap, seal it in an airtight container, and store it in the refrigerator. Briefly steam the plum pudding again before serving.