Peel the quince and apple, cut into quarters, core and finely slice.
Put the sugar and juices in a medium saucepan over a low heat. Stir to dissolve the sugar, increase the heat and bring to the boil. Add the fruit, cover with greaseproof paper or a lid and simmer for 15-20 minutes, or until very soft. Set aside to cool.
Pour everything into a liquidiser and purée until smooth. If the purée seems a little thick, add a splash of water. Set aside in a pan, then re-heat gently when ready to serve.