Put the fruit rinds in a saucepan. Make
the juice up to 600 ml/1 pt with water and add to the pan. Add the
sugar and stir over a gentle heat until the sugar has dissolved
completely. Bring to the boil and boil rapidly until the syrup has
thickened and reduced but is not colored. Lift out the fruit rinds
with a draining spoon and discard. Whisk the butter into the sauce, a
knob at a time, until thickened and smooth. Serve hot.