1.Peel oranges and lemons thickly; slice peel thickly, chop flesh coarsely. Discard seeds.
2.Combine peel, flesh and the water in large saucepan; bring to the boil. Reduce heat; simmer, covered, about 1 hour or until rind is soft.
3.Measure fruit mixture, allow ½ cup of each sugar for each cup of fruit mixture. Return fruit mixture, sugar and treacle to pan; stir over high heat, without boiling, until sugar dissolves. Bring to the boil; boil, uncovered, without stirring, about 40 minutes or until marmalade jells when tested. Stir in whisky.
4.Pour hot marmalade into hot sterilised jars; seal immediately. Label and date jars when cold.