Thoroughly wash the oranges and lemon, then cut in half and squeeze out the juice. Take a teaspoon and scrape away all the flesh and some of the pith of the orange halves. Place a sieve over a bowl lined with a muslin. Tip all the pips, pith and flesh into the muslin (you can use a clean jcloth). Tie with string and place in a large pan, tying the string to the handle. Add 2 litres of water.
Discard the lemon shell. Now finely cut the peel into strips; add to the pan.
Bring the mixture to the boil, then reduce the heat and simmer, uncovered, for 1.5-2 hours, or until the peel is very soft and the liquid has reduced by approximately half. Remove and discard the muslin bag, squeezing as much liquid as possible back into the pan between 2 plates.
Put a saucer in the freezer to chill. Add the sugar to the pan; stir over a low heat until dissolved. Turn up the heat and boil rapidly until it reaches setting point – (if you have a thermometer 102-105°C), about 15 minutes. To test, remove from the heat and spoon a little onto a chilled plate. Let it cool for a few seconds, then push with a finger. If the surface wrinkles, it’s ready; if not, boil for 5 minutes more and test again. Leave to settle for 30 minutes (this will prevent the orange rind floating to the top of the jar). Skim off any foam with a ladle. Stir and pour into warm sterilised jars. Seal when cool, then store in a cool, dark place for up to 3 months. Once open, store in the fridge for up to a month.