In a large bowl, mix the yogurt, curry powder, apricots, coriander, chilli (if using) and half the lime juice. Add the chicken pieces and stir to coat in the sauce, seasoning to taste.
Halve then slice the avocado and place in a shallow dish with the rest of the lime juice, tossing to coat.
Divide the chicken mixture in 2 and spread equally over 2 slices of bread. Top with the mango slices, avocado and a couple of leaves of the lettuce. Put another slice of bread on top and wrap tightly in cling film or greaseproof paper. Keep wrapped in the fridge until serving.