Score the skirt steak in fine parallel lines across the grain on both sides. Combine the citrus juices, achiote paste, half the garlic and 2 Tbsp extra virgin olive oil in a bowl and mix well until smooth. Add the steak, mix to coat, then cover and set aside for 30 minutes to marinate.
Arrange the corn over a hot barbecue grill and cook for 10 minutes, turning regularly, until lightly blackened. Use a sharp knife to remove the kernels, then mix with the tomatoes, cucumber, onion and coriander. Season with salt and pepper.
Mix the avocados, cumin, remaining garlic and remaining olive oil. Season with salt. Toast the tortillas over the barbecue grill until crisp.
Drain the steak, reserving the marinade, then cook over a hot barbecue grill for 6 minutes, turning several times and basting regularly with the reserved marinade, until medium-rare. Set aside to rest for 3 minutes, then slice finely. Arrange the avocado mixture, steak, salsa and curd onto the tortillas, then serve with lime wedges and hot sauce.