Preheat the oven to 350°F, 160°C-170°C, or gas mark 3–4.

In a large pot over medium heat, boil potatoes until tender, drain well and mash until creamy.

In a mixing bowl, add the mashed potatoes, butter, sugar, and oil and blend well with a whisk or hand mixer.

In a separate bowl, whisk eggs.

In a small bowl or mug, mix the custard powder with 1 cup of milk, and add to the whisked eggs.

Add the remaining 3 cups of milk to the egg mixture and stir well.

Combine the egg mixture with the mashed potato mixture, and stir well.

Pour half of the batter into a well greased 11" x 15" (30 x 40cm, or similar) oven roasting pan.

Put the half-filled dish into the oven, in the center of the middle rack, and add the remaining batter.

Bake at 350°F, 160°C-170°C, or gas mark 3–4, for 1 hour.

Cover the pudding with foil, and bake for a further 15 minutes.

Remove from the oven and let pudding cool and set at room temperature for a minimum of 6 hours, or overnight.

Serve warm or cold, with mixed berry compote, or stewed dried fruit.