Put the raspberries with the vinegar into a preserving pan and simmer until the fruit is a pulp. Strain, and press all the pulp except the seeds back into the pan. Add the salt, mustard and the mixed spice. Simmer gently for 20 minutes, then strain again. Add 6 oz. granulated sugar to each pint of liquid, allow sugar to dissolve over moderate heat, then boil quickly until thick. Pour into hot sterilized jar, seal. Serve with any cold cooked meat.