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Put the onions and celery into a food processor and whizz until finely chopped. Heat the oil in a very large saucepan, add the onions and celery, cover, then soften over a low heat for 5 mins. Add the garlic, cook 5 mins more, then tip in the spices and cook for 1 min.
Now stir in all remaining ingredients and bring to the boil. Keep on a bubbling simmer, uncovered, for 1 hr until the tomatoes are squashy and the liquid has reduced by several inches. Discard the cinnamon stick.
Whizz the mix with a stick blender until smooth, then sieve into a bowl. The ketchup will thicken a little when it cools, but if yours seems very runny (this will depend on the juiciness of your tomatoes), put it back on the heat and cook a little longer, stirring often, until reduced. Keep the ketchup in an airtight container in the fridge for up to 3 months, or freeze in batches. If you like, keep in sterilised bottles or jars for up to 6 months - see tip, below.