Tomatoes may be skinned if desired but it is not necessary. To skin tomatoes, prick the skin a little with a sharp knife and immerse them in boiling water for 15 seconds. Once you've taken them out, you can slide the skin off the tomatoes. Chop tomatoes and slice onions. Cover with water and salt then leave them overnight. Pour the brine off the tomatoes then almost cover them with vinegar. Bring to boil and add sugar, stir well. Mix curry powder, mustard and cornflour in a cup with some cold vinegar and stir into the simmering tomatoes. Continue stirring until mixture thickens. (If you find its not thick enough add more cornflour but take care to blend well with more vinegar and stir in quickly to prevent lumps forming) Simmer for around 30 - 60 minutes. Add in the remaining ingredients and stir well. Carefully pour into hot sterilised jars and seal. (I like to process them in a Fowlers Vacola unit as per unit instructions as well.)