Zingy ginger and grapefruit sponge pudding

Ingredients

• 160 grams butter
• 75 grams sugar
• 1 egg
• 1 1/2 cup self-raising flour
• 1 teaspoon ground ginger
• 1/2 teaspoon mixed spice
• 1 teaspoon baking powder
• 1/3 - 1/2 cup milk
• 1 grapefruit, segmented
• additional grapefruit slices, to decorate (optional)
• Grapefruit sauce
• 1 tablespoon butter
• 3/4 cup brown sugar
• 2 tablespoons golden syrup
• 1 cup boiling water
• 1/2 cup grapefruit juice
• 1 tablespoon cornflour
• 1 tablespoon cold water