Place a small side plate in the freezer for testing later.

Squeeze the juice from the orange slices using your hands. You should get roughly 500 millilitres. Combine with the lemon juice and set aside.

Add the squeezed orange slices to a medium, heavy-based saucepan and cover with water. Bring to a boil, then simmer for 30 minutes. Drain and allow to cool slightly.

Finely chop the softened orange slices. Remove any seeds if needed.

Return the chopped oranges to the pot, add the sugar and the reserved juice, and bring to a gentle boil.

Reduce to a very low simmer and cook for 3 to 3½ hours, stirring occasionally. The marmalade should darken and thicken.

Test readiness using the cold plate or check the temperature reaches 104°C.

Stir in the whisky and let cool slightly before transferring to sterilised jars.