See here for a visit review A section of Corton beach 2 miles north of Lowestoft. Officially designated in 1979
Beach is clearly marked with Signs.
Update : From Shake : June 2012 It is no longer "clearly marked with signs" and hasn't been now for several years, as the need for a 'clothing-optional' part of the beach became redundant in 2009.
Before the de-designation the nudist bit was just north of Tramps Alley (an unmarked walkway down from Corton Lane to the beach, opposite the putting range and (also unsigned) free carpark) and that remains the stretch where naturists gather today.