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Place the whole oranges and lemon juice in a large preserving pan and cover with 2 litres/ 4 pints water. If this is not enough to cover the fruit, put it in a smaller pan. If necessary, weight the oranges with a heat-proof plate to keep them under the water. Bring to the boil, cover and simmer very gently for about 2 hours, or until the peel can be pierced easily with a fork.
Warm half of the white and dark sugar in a very low oven. Pour off the cooking water from the oranges into a jug and tip the oranges into a bowl. Return the cooking liquid to the pan. Leave the oranges to cool until they are easy to handle, then cut them in half. Scoop out all the pips and pith and add these to reserved orange liquid in the pan. Bring to the boil for 6 minutes then strain this liquid through a sieve into a bowl, pressing the pulp through with a wooden spoon; the result is high in pectin, which helps to ensure the marmalade has a good set.
Pour half this liquid into a preserving pan. Cut the peel into chunky shreds, using a sharp knife. Add half the peel to the liquid in the preserving pan with the warm white and dark muscovado sugars. Stir over a low heat until all the sugar has dissolved, then bring to the boil and bubble rapidly for 15-25 minutes until setting point is reached. Stir in half the whisky.
Take the pan off the heat and skim any scum from the surface. (To dissolve any excess scum, drop a small knob of butter on the surface, and gently stir.) Leave the marmalade to stand in the pan for 20 minutes to cool a little and to allow the peel to settle, then pot in sterilised jars, seal and label. Repeat for the remaining batch.