Cook the sweet potatoes in a pan of boiling water for 10-12 minutes until tender. Meanwhile, put the tomatoes, chickpeas, chilli flakes and curry paste in another pan and simmer gently for about 5 minutes.
Preheat the grill. Drain the sweet potatoes and add to the tomato mixture. Stir in the spinach and cook for a minute until just starting to wilt. Stir in the coriander, season to taste and keep warm.
Sprinkle the chapatis with a little water and grill for 20-30 seconds each side. Spoon on the filling, top with yogurt and fold in half to serve.