Layer the tomatoes and cucumber in a large bowl, salting each layer. Cover and leave to stand for 24 hours. Pour the salted cucumber and tomatoes into a colander, rinse under cold running water and drain well. Put them into a large stainless-steel or enamel preserving pan and add the capsicums, sugar, mustard powder, grated nutmeg, mustard seeds and vinegar. Bring to the boil, then reduce the heat and simmer, stirring frequently, for 30 minutes, or until the tomatoes are softened and the mixture is reduced by a quarter.
Spoon the relish into clean, warm, dry jars and cover them immediately with vinegar-proof lids. Label and date when cool. Store in a cool, dark, airy cupboard and use within two months. Once opened, keep refrigerated and use within a week.