For the raita, mix the radishes, spring onions or chives, garlic, cucumber and yogurt. Season with salt and ground black pepper and refrigerate until needed.
Heat the flour tortillas in a microwave according to the packet instructions. Spread each tortilla with a large spoonful of the raita and then top with the sliced roast beef. Add a handful of watercress and the tomato pieces. Roll up the tortillas, cut in half and serve.
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