Bring a large pan of salted water to boiling, add the dried pasta and cook for 8 to 10 minutes until al dente. Wash the watercress and trim the ends of the stalks; pat dry on kitchen paper. Put into a food processor with the garlic and blend until finely chopped. Add the nuts and process briefly until they are chopped. Add the Cheddar cheese. While the processor is running, slowly add the oil. Drain the pasta, reserving 2 to 3 tablespoons of the cooking water. Return pasta to pan. Add the pesto, with a little of the water to loosen it up, and toss together. Serve with extra cheese crumbled on top.
*Halve some cherry tomatoes, place on a foil-lined grill pan and drizzle with oil. Season, then broil for 5 minutes. Toss with the pasta before serving.