Preheat the oven to 180C/fan 160C/gas 4.
Firstly, prepare the duck by placing it breast side down on a chopping board. Using a strong pair of scissors, cut down either side of the backbone from neck to tail. Remove the backbone and save for stock, or discard. Gently prise apart and push down on the breastbone to flatten. Turn the duck over so breasts are facing upwards.
Prick the skin around the legs and then season the duck on both sides with salt, freshly ground black pepper and the smoked paprika.
Place the duck on a baking tray, breast side up and place in the oven. Cook for 1 hour 30 minutes.
While the duck is cooking prepare the piri piri sauce - place the garlic, smoked paprika, chillies, oil, sugar, vinegar, lemon zest and juice into a blender and blitz. Use this sauce to baste the duck 5 or 6 times while it's cooking.
Approx. 30 minutes before the duck is due to be ready, take a deep sided baking tray, big enough to hold the potatoes and pour in the duck fat. Place the tray into the oven and after 10 minutes, once the oil is hot, take it out and carefully put your potatoes into the tray. Gently sprinkle the cumin seeds plus some salt and freshly ground black pepper over the potatoes and return to the oven. They will be ready in 20 to 30 minutes.
When the duck is cooked, remove from the oven and leave in a warm place to rest for a few minutes, pour over any remaining sauce or serve it on the side.
When you are ready, serve the duck with the potatoes along with some vegetables or a mixed salad.
Optional - for a chargrilled finish place the duck on a hot large griddle pan at the end or in the summer, place on the barbecue.