Preheat the oven to 200°C/fan 180°C/gas mark 6. Thread the potatoes on to metal skewers. Using a sharp knife, carefully cut the potato in a spiral all the way round, starting at the top of the potato and rotating the skewer as you cut at an angle. When you've cut the spiral, gently fan the potato out in a corkscrew.
Mix the oil with the cumin, paprika and chilli flakes. Brush two thirds of the spice mix over the potatoes. Place on a baking tray and bake in the oven for around 20-25 minutes. Remove from the oven and baste with the remaining spice mix. Finish the potatoes on the barbecue for 5-10 minutes until the potatoes are tender and golden.