Heat the olive oil in a large saucepan over medium heat. Stir in the onion, garlic, cumin, and curry powder; cook and stir until the onion has softened and turned translucent, about 5 minutes. Add the chickpeas, red pepper, turnip, sweetcorn, and tomato sauce. Season with chilli flakes, salt, and black pepper. Bring to a simmer over medium-high heat, then reduce heat to medium-low, cover, and simmer until the vegetables are tender and the curry has thickened, 1 1/2 to 2 hours.