Cornwall's always good for waves! The best time for waves in cornwall is the winter, it mite be chilly but the swells are bigger and the waves offer more of a punch, also there aren't so many poeple out which means more waves for everyone. September/October maybe more suitable as the water remains warmer.
Marazion works well when other beaches are blown out, but works best in W, E and SE. Easy to find, loads of space, and stunning views....you can also buy an awesome pasty in the bakery just up the road!
Try Coverack Windsurfing Centre for kit/tuition/advice, they're a good bunch.
Happy sailing!