1.Peel lemons; chop rind coarsely. Chop flesh coarsely; reserve seeds and juice. Peel grapefruit, chop rind into fine strips; reserve. Chop flesh coarsely; reserve seeds and juice.
2.Combine lemon and grapefruit flesh with reserved juice in large saucepan. Tie reserved seeds, lemon rind and fresh peeled ginger in piece of muslin, add to pan. Add grapefruit rind with the water and acid; bring to the boil. Boil, uncovered, about 1½ hours or until rind is soft and mixture has reduced by half. Discard muslin bag.
3.Add sugar; stir, without boiling, until sugar dissolves. Boil, uncovered, without stirring, 15 minutes or until marmalade gels. Add glacé ginger; stand 10 minutes.
4.Pour into hot sterilised jars; seal immediately.