Heat the oil in a large pan. Throw in the mustard and cumin seeds and stir until they begin to pop. Add the onion,garlic,celery,and chilli and cook for a further 2 minutes add the squash,water and creamed coconut,bring to the boil and simmer,stirring occasionally,until the squash is really soft and tender. Transfer to a blender and process until smooth. Season to taste and serve. Or cool and refrigerate,ready to reheat and take to work in a flask the next day