Preheat the oven to 230 degrees C. Rub the fillet with the olive oil and set on a rack in a roasting tin. Roast for 15 minutes for rare beef or up to 25 minutes for well done.
Meanwhile, whisk together all the ingredients for the vinaigrette in a large mixing bowl.
Remove the beef from the oven and leave to stand for 5 minutes, then cut into thin slices against the grain. Add to the dressing and leave to cool.
While the beef is cooling, cook the potatoes in a saucepan of boiling water for 15 minutes or until tender. Drain well. When cool enough to handle, cut them in half or into thick slices and add to the bowl with the beef.
Drop the green beans into another pan of boiling water and cook for 1 minute. Add the peas and continue cooking for 3 minutes or until the vegetables are tender. Drain and refresh briefly under cold running water, then add to the beef and potatoes. Toss well. Cover and chill.
About 15 minutes before serving, remove the salad from the refrigerator and stir in the leek and chives.