In reply to AdCo82:
Trebarwith is great, but disappears at high tide.
Moving southwards, If you don't mind a short walk, Tregardock (should be running past it tomorrow if the will power holds up) is expansive and bound to be emptyish, then you have Lundy Bay, a short walk from Polzeath, also tidal, also lovely. I went past that a couple of days ago. If you think you would find Polzeath itself too crowded, the beaches around from it and on the Padstow side are also really nice - a bit more empty and sheltered, and still a short walk from either Polzeath itself, or Rock, or Padstow by ferry. Yesterday I ran around the head up from Padstow, to Stepper Point and round to Trevone. Daymer Bay beach, the one opposite it on the Padstow side and Trevone itself would all fit your bill.
....and if you can get away from the north coast, try the Roseland on the south coast. Any of the beaches between St Anthony's Head and Nare Head eg Towan, Porthcurnick (this one, especially - it has the Hidden Hut just above it, and Portscatho just a hop away) and Pendower are all quiet and beautiful, and a world away from Fistral.
Post edited at 15:29