1) If using regular yoghurt place the yoghurt in a sieve lined with kitchen paper. Put the sieve over a bowl and place in the refrigerator to drain and thicken for 30 minutes.
2) In a small bowl, combine the thickened or Greek-style yoghurt, mayonnaise, lemon juice, mustard, salt and thyme until smooth. Fold in chicken, grapes, walnuts and apples. Season with pepper.
3) Place a lettuce leaf on a wrap. Spoon about 100g of the chicken filling onto wrap and roll wrap around filling.
Slice in half and serve.