Cook the pasta in a large saucepan of lightly salted boiling water for 10 minutes. Add the broad beans and bring back to the boil, then reduce the heat slightly and cook for a further 2–3 minutes until the pasta is tender and the beans are lightly cooked.
Meanwhile, mix together the tomatoes, ham, chives and oil in a large serving bowl. Set aside. To make the pesto, process the watercress, walnuts and garlic together in a food processor or blender until finely ground. Add half the oil and pulse until combined, then add the remaining oil, lemon rind and juice and the yoghurt and blend again. Season to taste.
Drain the pasta and beans and immediately add them to the tomato mixture. Mix well, then cover and leave to stand for 5 minutes. Stir the pesto and chopped watercress into the salad, then serve immediately.