This recipe worked well for me.

I made eight jars.

1.25kg Seville oranges
5 -7 cm piece ginger
1.5kg granulated sugar (unrefined if possible)
1x 200g jar preserved stem ginger

Wash the oranges and, if they are not organic, scrub them using warm water to remove wax. Put them into a preserving pan with 2.25 litres water and a scrubbed length of ginger (no need to peel).

Bring to the boil and then let cook gently with a lid on for about 40 minutes or until the oranges are soft. Leave to cool. Using a large slotted spoon, remove the oranges and cut in half. Scrape the insides into a bowl. Cut the peel into fine slices and return it to the preserving pan.

With a little bit of juice added, blitz the piece of ginger and place this in the bowl together with the pith, seeds etc from the oranges and turn into a muslin cloth. Tie the cloth, hang it and let the juice strain for at least two hours and preferably overnight.

Add the strained juice to the preserving pan along with the sugar. Bring to the boil slowly and stir frequently to make sure the sugar is melted. Let it boil for 10 minutes.

Blitz the contents of a jar of preserved ginger and add this to the pan.

Let the marmalade boil gently for a further 30 minutes before checking to see if it has reached setting point. Keep checking every 10 minutes until you are satisfied.

Pour into warmed sterilised jars.