Cut each orange into 6 wedges, then thinly slice. Tie the orange pips in a square of muslin. Add oranges and pips to a preserving pan, pour over the measurement water and add the lemon juice. Bring slowly to the boil, then simmer gently, uncovered, for about 1½ hours until reduced by almost half.
Add the sugar and heat gently, stirring occasionally, until dissolved. Bring to the boil, then boil rapidly until setting point is reached (10–20 minutes).
Lift out the muslin bag, squeezing well. Skim with a draining spoon or stir in butter if needed. 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, this marmalade goes beautifully with sliced walnut bread.