Put the sugar, slices of orange and the tea bag into a saucepan and add 5 cups of water.
Heat, stirring, until the sugar has dissolved.
Add the prunes to the saucepan and cover the pan with a lid.
Simmer very gently for 1 1/2-2 hours, until the prunes are well plumped up.
Cool, then pour the prunes and their syrup into a glass bowl.
Cover and chill until ready to serve.