In a large saucepan, heat the oil and sauté the onion and garlic until soft and transparent. Add the smoked paprikas, cumin and cayenne, and continue to cook for a further minute, then stir in the sherry, vinegar and tomatoes. Bring to a simmer and cook to reduce by a third. Cool slightly, then purée in a food processor until smooth. Remove to a bowl, season with salt to taste and leave to cool. When cold, combine with the mayonnaise. Season potatoes with salt and steam or fry until tender. Transfer to a bowl and fold in the desired amount of sauce (you may not need it all). Garnish with a sprinkling of paprika and chervil or parsley and serve.