Scrub the oranges, remove the buttons at the top of the fruit then cut in half around their circumference.
Squeeze out the juice and keep to one side.
Slice the fruits into the size pieces you prefer removing any marked skin and any thick pieces of the white inner pith. Save these – the pith is where the most pectin is – tie these off-cuts in a square of muslin.
Place the sliced peel, orange juice, pith bag in a large bowl and cover with 2 litres of water.
Cover and leave to soak overnight or for up to 24 hours – this helps to soften the peel and release the pectin.
Transfer the whole mixture to a large heavy based pan or preserving pan.
Cover and bring to the boil, then simmer for about 2 hours (Sevilles have tough old skins) or until the peel is tender and breaks when gently pulled – the contents of the pan will have reduced by approximately one- third.
Remove the pith bag, first squeezing it firmly against the side of the pan to remove all its gummy goodness.
Add the lemon juice and the sugar. Bring the mixture to the boil, stirring until the sugar has dissolved.
Boil rapidly until setting point is reached –this should take approximately 15 minutes at a full rolling boil but will depend on the size of pan you use and how hard the boil is. The marmalade is done when the mass of foamy bubbles on the surface have disappeared and the mixture appears to be thick and glossy.
Test for setting point by dropping of the mixture onto a very cold plate or a large stainless steel spoon – after a minute or so it should form a slight skin on the surface. Avoid over-cooking which results in a stiff overly sweet marmalade.
Remove from the heat and leave to cool for several minutes to allow the peel to evenly distribute – if you pour when the marmalade is too hot the peel will float to the top of the jar. Pour (to within 3 mm of the top) into sterilised jars and seal immediately.
Makes about 4 x 500ml jars.