Prepare your glass jars by washing thoroughly in hot, soapy water. Rinse, drain and then place in warm oven 50°C after you have cut the orange peel prior to cooking your marmalade.
Wash the oranges. Place the coriander on a square of muslin, and tie with string. Place the oranges and coriander in a large pan, cover with the water. Bring to the boil, reduce heat to simmer and cook for approximately 2 hours until the orange peel is tender. Dispose of the coriander.
Remove the oranges and reserve the cooking water. Measure the cooking water and make up with additional water, or reduce the water if necessary to 2.25 pints. Add the sugar to the cooking water and stir over a very low heat until fully dissolved. Add the lemon juice.
When the oranges are cool cut in two, remove pips with a fork, and scoop out the fruit flesh and pith with a soup spoon. Cut the peel into strips approximately .75cm x 4cm. Add the fruit flesh, pith and peel to the sugared orange water. Stir and bring to a ‘rolling’ boil and boil rapidly for approximately 10-15 minutes.
Put a preserving thermometer into the marmalade and when the temperature reaches 102.5°C – 104.5°C it should set once cool. When the temperature reaches 102.5°C, take the marmalade off the heat. Put a very small amount of marmalade onto a cold saucer (that has been in the fridge) and allow the marmalade to cool on the saucer.
Gently push the marmalade with your fingertips and if it crinkles, the setting point is reached. Boil and test again as necessary if the marmalade doesn’t crinkle. Do not cook beyond 104.5C.
Leave the marmalade to cool in the pan for 10 minutes stirring gently to disperse the peel and any scum. Ladle the jam into a measuring jug, before filling and lidding each jar.
Store in cool dark place for up to 12 months.