Peel and chop the sweet potatoes. Place in a large saucepan, add cold water to cover and season generously with salt. Bring to a boil, lower the heat, and simmer until potatoes are fork tender.
Heat the oil in a large, heavy based pan. Finely chop the onion and garlic and cook in the pan until softened.
Drain and tip the sweet potatoes, chickpeas and spices into the pan and stir well. Cook for a 3-5 minutes then stir in the tomatoes, stock and coconut milk. Bring the mixture to the boil then reduce the heat and simmer for 25-30 minutes.
Stir the spinach leaves into the mixture and cook for further 10-15 minutes. If there is excess liquid, simmer for a little longer. Remove from the heat and leave to cool.
Preheat the oven to 180C/Gas mark 4 and brush a large ovenproof frying pan with oil. Alternatively use a loose-bottomed cake tin.
Brush around 5 of the filo pastry sheets with oil and layer them in the pan/tin so that the whole thing is covered evenly, with the sides hanging over.
Spoon in the spinach and sweet potato mixture and spread out over the base. Fold in the overhanging filo to form a lid. Scrunch up the remaining sheets and arrange on top to cover the top of the pie. Brush with a little oil and bake in the centre of the oven for 35-40minutes or until the filo is golden and crisp.