Seville Orange Marmalade

My favourite marmalade recipe, the one I use year after year comes from Jane Grigson’s beautiful “Fruit Book” first published by Michael Joseph in 1982: the simplest, easiest and best-flavoured marmalade, she says, I agree.
Scrub 1 ½ kilos of Seville oranges and put them in a pan with 3 ½ litres of water. Simmer until skin is tender, about 1 ½ hours. Take oranges out of the water, cool, halve and remove the pips. Put the pips in a piece of muslin. Cut up the orange flesh or pulse in a liquidiser being careful not to reduce it to a mush. Return shredded fruit to water. Add 3 kg of preserving sugar and hang the little bag of pips over the side of the pan on a piece of string so they bubble along with the fruit and release their pectin. Stir gently over heat till sugar dissolves. Bring to the boil and boil vigorously till setting point is reached. Test for setting by placing a tiny spoonful of syrup on a cold saucer and putting it in the fridge or freezer for a few minutes. If setting point is reached a wrinkled skin will form.
Leave the marmalade to stand for 15 minutes to allow peel to settle, remove the bag of pips and discard, then pot the warm marmalade into warm sterilised jars and cover: so nice!