Heat the oil in a saucepan, add the diced chicken and onion and fry for 5 minutes, stirring until golden. Add the chorizo and garlic and cook for 2 minutes.
Mix in the tomatoes, stock, capers and seasoning, bring to the boil then cover and cook gently for 30 minutes, stirring from time to time, until the sauce has thickened slightly and the chicken is cooked through. Allow to cool then chill until required.
When ready to serve, cook the pasta in a large saucepan of boiling water for 7–9 minutes until just tender. Meanwhile, add the olives and half the basil to the chicken and reheat thoroughly until piping hot. Drain the pasta, toss with the chicken mixture then spoon into dishes and garnish with the remaining basil leaves.