Place the cubed potatoes in a large saucepan and cover with water. bring to the boil and simmer for 5 to 7 minutes. Drain the potatoes and set aside.
Fry the onion with the olive oil for about 10 minutes on a low heat until they become translucent. Add garlic and chilli and fry for another 2 minutes, but be careful not to burn.
Add the paprika and chilli flakes, then fry for another minute or so. Add chopped tomatoes, water, sugar, salt and pepper and simmer for about 20 minutes on a really low heat.
Meanwhile vegetable oil in a large frying pan and fry the cubed potatoes until they are nice and crispy and cooked all the way through.
Serve in a big bowl, pouring the sauce over the potatoes.