Rinse the sultanas and place in a large saucepan with the sugar, cinnamon, oil and water.
Cook over a very low heat for around 45 minutes, stirring from time to time; the sultanas should not be too coloured.
Meanwhile, dry-toast the walnuts over a low heat for 5 minutes, then remove and leave to cool. When they are cold, roughly crush them in your hand and add them to the sultanas 5 minutes before the end of cooking.
Tip into a preserving jar and leave to cool.