Peel the fruits with a vegetable peeler, then cut the rind into very fine shreds. Add to a saucepan with 600 ml (1 pint) water, bring to the boil, then cover and simmer gently for 20 minutes until the rinds are tender.
Meanwhile, roughly chop the peeled fruits, put into a preserving pan with 1.2 litres (2 pints) water, cover and simmer gently for 1 hour until soft.
Strain the fruit mixture through a fine nylon sieve or jelly bag into a large bowl, pressing the pulp to extract all the juice. Return to the preserving pan, stir in the cooked fruit rinds and cooking water and reheat.
Add the sugar and heat gently, stirring from time to time, until the sugar has dissolved. Bring to the boil, then boil rapidly until setting point is reached (10–20 minutes). Skim with a draining spoon or stir in butter if needed.
Leave to cool for 10 minutes, then 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.
To serve, try spreading this marmalade over a lemon tart.