Whisk the egg white with a little water.
Put the quince and apple juice, the water, a little lemon juice and the rind, sugar, egg white and the egg shell into an enamel saucepan. Over a gentle heat, whisk the mixture until it comes to the boil; skim.
Meanwhile, soak the gelatine leaves in cold water for 5 to 7 minutes; remove the gelatine mass and gently squeeze out the water.
Pour a kettle of boiling water through a jelly bag (or a piece of muslin or a clean tea-towel) to warm it. When the water has drained off, pour in the jelly and let it strain into a wetted mould. (I put the wetted jelly mould in a large saucepan, placed the sieve across the top and draped the warm wet muslin across the sieve.) Refrigerate the jelly for at least 3 hours, or overnight, to set.