Peel the potatoes and cut into pieces. Quarter the cucumber lengthways and remove the core. Then cut the cucumber into 5 mm cubes and mix with the yoghurt. Season to taste with salt and a little lemon juice.
Heat a pan and fry the potatoes in oil. The pan should not be too hot (slightly above medium heat) if the potatoes are still raw. If the potatoes are pre-cooked from the day before, for example, the heat can be increased a little further. Fry the potatoes until they are brown on all sides.
As soon as the potatoes are brown on all sides, add the Bombay curry powder and fry briefly while turning. Add a small spoonful of butter and turn the potatoes until the butter has melted.
Arrange the Bombay fried potatoes on a plate. Place the cucumber raita in a bowl, garnish with chilli flakes and serve.