You can scale this up, as you wish. Each batch makes about 2½ kg, c. 5lbs of marmalade. There are a couple of different ways of doing it, both a hassle, but worth it in the end.
1 1kg bag Seville oranges
1 kg preserving or granulated sugar
1 litre water (or less, depending which method you use).
Place the whole fruit and the water in a pressure cooker, bring to pressure and cook for 20 minutes at high pressure. Allow to cool. When fruit is cool enough to handle (you can leave it overnight, of course), cut each piece in half, scoop out the insides and return them to the pan, and chop the peels very finely. Boil the insides in the pan for 5 minutes, then strain to remove the pips. Add the chopped peel and sugar, and proceed as below.
Cut fruit in half and juice it. Measure the juice, and make up to 1 litre with water. Boil the pips with some of this liquid (about 150 ml) for 5 minutes, and make up the jug to 1 litre again. Strain the pips. Meanwhile, you have been chopping the peels, which is a lot harder when they are not cooked, but you save time by not having to wait for it to cool once the pressure cooker has lost pressure. Boil the chopped peel in the juice at high pressure for 20 minutes, allow to cool at room temperature, and then proceed.
Put the sugar into a large pan and add the cooked fruit/water/juice mix. Stirring all the time, heat gently until it comes to the boil, then allow to boil, stirring frequently, until setting point is reached, which you test on a plate you had previously put in the freezer. "When it gels, it's jam" to quote Elizabeth Goudge. Allow to sit for ten minutes, then stir, pot in glass jars which you have sterilised in a warm oven while all this has been going on, and seal.
For Susan Gerules.
Oh bum, just discovered I already posted this recipe back in 2012.... oh well. I could delete this, I suppose, but I've written it now....