All those are adding twice the weight of sugar to oranges which I find way too sweet.From different books, all Sevilles -
1kg oranges
2l water
2kg sugar
juice of three lemons
..............
1KG oranges
1 lemon
2kg sugar
2.25l water.
................
450gm oranges
1.2l water
900gm sugar
................
1 1/2lb oranges
2pt water
juice of one lemon
3lb sugar.
..............
Much of a muchness. One includes a teaspoon of salt. And I can't find the book I was looking for.
I only add 1.25 x the weight of oranges, plus all the orange goes in and the only bits discarded are the pips and the ends and that + two whole lemons makes for a nice tangy marmalade.
Also in my 30+ years of making it to this recipe I've never had an issue with mould even with 2y old jars, and even opened ones which I never even store in the fridge -maybe the relatively high acid content protects it.
..but I guess each to their own as to recipe you follow -am sure those that make their own would never consume shop-bought again