Put the vinegar and mixed spice in a stainless-steel or enamelled saucepan. Bring to the boil and remove from the heat. Cover and eave to infuse for 3–4 hours.
Put the tomatoes in a separate heavy saucepan over a medium heat, and simmer gently until pulpy.
Rub the tomato pulp through a sieve into a bowl. Return the strained pulp to the cleaned pan. Add the salt and simmer gently until the mixture thickens. Add the sugar and infused vinegar, stirring until completely dissolved.
Continue simmering, stirring occasionally, until the mixture is the consistency of whipped cream. Pot into hot sterilized glass jars, and seal tightly (make sure the lids are vinegar-proof). The tomato sauce will keep for up to 6 months.