Cook your pasta in salted boiling water for 15 minutes until they are just cooked through.
Keep a few leaves aside. Put pine nuts and the garlic in a food processor, chop finely. Place the majority of the watercress and cheese in the processor and blend again. Add your olive oil and then the lemon juice. Season to taste.
Drain your pasta in a colander. Place the pasta back in your pan, with the pesto, tomatoes and olives. Scatter the watercress you put to one side on top and serve.