Heat the butter in a large bowl set over a saucepan of simmering water until melted.
Pour the sugar into the melted butter, then add the grated fruit rinds. Strain in the fruit juices, discarding any pips, then strain in the beaten eggs and mix together. Cook for 40–50 minutes, stirring from time to time, until the sugar has dissolved and the mixture has thickened.
Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool. Store in the refrigerator for up to 2 weeks.