Crush the seeds roughly in a pestle and mortar, then toast in a hot preserving pan with the chilli flakes for a few seconds. Add all the remaining ingredients, then cover and simmer gently for 30 minutes, stirring from time to time.
Uncover the chutney and cook for 30 minutes, stirring until thick and pulpy. Mash with a potato masher, or blitz in a food processor or liquidizer, until smooth.
Ladle into warm, dry jars, filling to the very top and pressing down well. Disperse any air pockets with a skewer or small knife and cover with screw-top lids. Label and leave to mature in a cool, dark place for at least 3 weeks.
To serve, try adding to a cheese and salad sandwich.