Place apples and water in a large saucepan. Bring to the boil, reduce heat and cook 25 minutes, or until apples are tender, stirring occasionally. Add more water as necessary to keep the apples simmering.
Wrap mustard seeds in muslin or cheesecloth and place with apples. Add onion, tomatoes, curry powder, cayenne pepper, salt, sugar, sultanas, garlic and vinegar in with the apples. Stir until sugar has dissolved.
Bring the mixture to the boil. Reduce heat, and simmer 3 hours, stirring occasionally, until a thick chutney remains. Remove and discard wrapped mustard seed. Seal chutney in sterile containers until serving.