Cook the pasta in a large saucepan of salted boiling water according to the packet instructions until al dente.
Meanwhile, put the pine nuts, garlic and watercress in a food processor with a generous pinch of salt. Process for 15 seconds until roughly chopped. Then, with the motor running, drizzle in the oil while you process for a further 20 seconds.
Drain the pasta thoroughly and tip into a bowl. Crumble in the goats' cheese and stir well. Season with pepper, then stir the pesto into the hot pasta. Divide between 4 serving plates and serve immediately with a scattering of goats' cheese and pine nuts and some watercress sprigs.