Soak the melon overnight in enough vinegar to cover.
Add onions, sugar, salt, mixed spice and cayenne then boil until tender, about 2 hours.
In a small bowl, mix together mustard, turmeric and plain flour. Add enough cold vinegar to make a paste then stir it into the melon mixture. Boil for 5 to 10 more minutes. Allow to cool before bottling.