First you need to skin the tomatoes, so pour boiling water over them and leave for exactly 1 minute before draining them and slipping off the skins.
Put the onion, chilli, garlic and tomatoes in a food processor and blend until finely chopped, then place the mixture in a saucepan and add the sugar and vinegar. Place the pan over a gentle heat and simmer very gently, without a lid, for 2 hours, by which time the mixture will have reduced to a thick sauce.
Towards the end of the cooking time, stir frequently so the sauce doesn't stick to the bottom of the pan. Then taste to check the seasoning. Cook the chips according to the pack instructions and serve with the relish, which can be hot or cold.