Photograph: Dan Jones
Bubble the tomatoes, chillies, vinegar, lemon juice and tomato purée together in a medium-sized pan for 8-10 minutes. The mixture should be thick enough that when you run a spoon through it, a gap remains in the middle.
Add the sugar and some seasoning. Stir until the sugar dissolves, then boil rapidly for 5 minutes until thickened, stirring occasionally.
Pour into a sterilised jar, cover and leave to cool. It will keep for several weeks in the fridge.
Kitchen secret: sterilise jars and bottles before filling by running through the dishwasher or washing, then drying in a low oven.