Begin by lightly buttering the base of the saucepan (to help prevent the marmalade catching) measure 2.5 litres of water into the pan.
Cut the lemon and oranges in half (discarding any bits of stalk) and squeeze the juice out of them. Add the juice to the water and place the pips and any bits of pith that cling to the squeezer on the square of gauze or muslin (laid over a dish).
Cut the orange peel into quarters with a sharp knife. One by one, fold and squeeze the quarters tightly together and cut them into shreds (easiest with a small serrated knife). Shreds can be either chunky or thin. As you cut, add the shreds to the water. Any pips or pith should go onto the muslin.
(The pith contains a lot of pectin so don't discard any and don't worry about any pith and skin that clings to the shreds – it all gets dissolved in the boiling. The lemon can be included or not).
Tie the pips and pith up loosely in the muslin to form a little bag. Tie this on to the handle of the pan so that the bag is suspended in the water.
Bring the liquid up to simmering point and simmer very gently, uncovered, for roughly 2 hours until the peel is totally soft (test a piece carefully by pressing it between finger and thumb: if you can squeeze it in half the peel is ready).
Meanwhile, chill three very small plates or saucers in the freezer compartment of the fridge.
Remove the bag of pips and leave to cool on a plate. Pour the sugar into the pan. Stir it now and then over a low heat until all crystals have dissolved. (Check carefully: dig in with a wooden spoon, lift it up - if you see any granules on the back of the spoon, give it a bit longer.)
Now increase the heat to its highest. Squeeze the bag of pips over the plate to extract all of the sticky, jelly-like substance that contains the pectin. As you squeeze you'll see it ooze out. You can do this by placing the bag between two saucers, but the best way is to use your hands.
Using a balloon whisk, whisk the jelly into the ingredients in the pan. As soon as the mixture reaches a fast boil, start timing. Stir occasionally. After 15 mins, remove pan from heat. Spoon a little marmalade onto one of the cold plates. Let it cool back in the fridge for a few mins.
When it has cooled, you can test if you have a 'set' by pushing the mixture with your little finger: if it has a crinkly skin, it is set. If not, continue to boil the marmalade and give it the same test at about 5-min intervals until it does set. You may need to test it 2 or 3 times.
After that remove the pan from the heat. If there's a lot of scum, most of it can be dispersed by stirring in half a teaspoon of butter, and the rest can be spooned off. Leave the marmalade to settle for 20 minutes.
In the meantime, the jars should be washed in mild soapy water, rinsed and dried and heated in a medium oven for 5 minutes.
Pour the marmalade, with the aid of a funnel and a ladle, into the warm jars, (so that it goes right up to the neck) cover with waxed discs (wax side down) making sure you cover all the surface and put the lids on while still hot.
Label when cold and store in a dry, cool, dark place.
