Bring the water for the rice to the boil in a separate pan. Add the rice and salt and stir well once. Cover, reduce heat and simmer for 10 minutes.
Meanwhile, heat the sunflower oil in a wok or large pan and gently fry the sweet potatoes, garlic, pepper, onion and chilli for 5 minutes.
To make the Colombo spice mix, add the ground cumin, ground coriander, caraway seeds, fennel seeds and tumeric in a pestle and mortar. Peel and mince the garlic cloves and finely chop the chillies and add it into the mix and pound the spices together to release the flavours. Sprinkle over the Colombo spice mix and cook for 2 minutes stirring well.
Meanwhile pour the coconut milk into the pan and combine thoroughly. Bring gently to the boil, lower heat and simmer for 10 minutes or until sweet potatoes are softened.
Add the prawns and coriander and stir to heat through. Simmer for 2-3 minutes only to prevent overcooking the prawns.
Serve immediately with the cooked basmati rice.