I use Delia Smith's recipe - even easier because it can be done all on the same day, works every time perfectly.
Here it is in case you want to consider it for next year.
DELIA SMITH'S SEVILLE ORANGE MARMALADE
Makes about 6lbs.
2 lb of Seville oranges 4 pts.(2.25 Litres) water l lemon 4 lbs.(1.8 kg) white granulated sugarSix 1lb (450g) jam jars
9 inch square cheesecloth 3 or 4 saucersMeasure water into large, heavy saucepan. Cut the oranges and lemon in half and squeeze the juice out of them. Add the juice to the water and put any bits of pith, flesh and pip in the cheesecloth square which is laid over a bowl. Now cut the oranges and lemon into quarters with a sharp knife and then cut the quarters into thinnish shreds. Add to the pan. As you cut, any spare pips and bits of pith go into the cheesecloth. (The pips and pith contain a lot of pectin and set the marmalade without having to add pectin, so it is necessary to do this)
Tie the cheesecloth up to form a little bag and leave six inches or so of the string to tie to the side of your pan (don't have to but it makes it easier to remove later) Bring the liquid up to the simmering point and simmer for a couple of hours until the peel is soft. Test a piece by squeezing betweem finger and thumb. At this point put your saucers in the freezer.
Remove the bag of pips and leave it to cool. Pour the sugar into the pan and stir it now and then over a low heat until all the crystals have melted. (check this carefully, its important.) Now increase the heat to high, and squeeze the bag of pips over the pan to extract all the sticky, jelly like substance that contains the pectin. As you squeeze you will see it ooze out. You can also do this by placing the bag between two saucers and squeezing.
As soon as the mixture reaches a really fast boil, start timing. After fifteen minutes spoon a little of the marmalade onto one of the cold saucers from the freezer and recool in the freezer a moment or so. You can tell - when it has cooled - if you have a "set" by pushing the mixture with your finger: if it has a really crinkly skin, it is set. If not, continue to boil and test again at about ten minutes intervals until it does set.
Remove the pan from the heat and allow to settle for about 20 minutes. This makes sure that the pieces of rind are evenly distributed throughout the jar when cold. Pot and cover. Best to leave a month or so before using.