Put everything into a large, preserving pan*, slowly bring to the boil, then simmer very gently for around 2-2½ hours, stirring occasionally to check how it is coming along and to prevent it sticking to the pan. It will continue cooking as it cools down, so don’t go too far, but do make sure that most of the vinegar has disappeared and it has a nice gloopy consistency.
Pour into sterilised jars while still warm, seal well making sure there is no contact with any metal, cool and label. Leave to mature in a cool dark cupboard for at least 2 months before eating.
The point of a preserving pan is its extra-wide top – this makes the evaporation go faster and reduces cooking time. If you are using a narrower pan it will take longer.