Wash the fruit thoroughly. Peel the grapefruit and oranges, making sure that you remove all the pith. Divide into boats and cut into small pieces. Peel, halve, and de-seed the star fruits, then cut into pieces. Thinly slice the kumquats. Put all the fruit in a pan with the water, cover and simmer for 10 minutes. Add the vanilla sugar, cardamom and jam sugar and boil for 3 minutes, stirring constantly. Skim off any scum. Pour the marmalade into warm, sterilised jars. Seal. Store in a cool place.
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