Heat the olive oil in a pan, add the shallots and garlic and cook for 3-4 minutes until soft but with no colour. Add the sugar and vinegar and stir in until the sugar is dissolved.
Bring to the boil and cook for 2 minutes. Combine the tomatoes, tomato ketchup, cornflour and cumin and add to the pan; reduce the heat and simmer for 15-20 minutes until thickened to