Toss the chicken with the lime zest and juice, pepper, oil and a little salt. Cook on a griddle pan or barbecue for 6-8 minutes, turning untill evenly cooked and nicely browned.
Heat the tortillas in the microwave, following pack instructions. Spread a little guacamole over each tortilla, top with some lettuce and chicken, then drizzle over the yogurt. Roll up and enjoy.
Mash the flesh of one ripe avocado in a bowl with a fork. Add 2 finely chopped tomatoes, 1/2-1 tsp hot chili powder (or you can use a fresh chilipepper which I did), then the juice of 1 lime or 1/2 lemon and a little salt and pepper. Stir everything together and serve.