Like at every beach, especially those with salt ponds/mangroves in the area (both Hermitage and Galley Bay) sand flies will be present, although frequent fogging does help to keep them at bay for the most part. The biggest determinant to the number of sand flies in the area will be whether or not there has been significant rain in the preceding days, so it's difficult to make a realistic judgement without knowing how much rain has fallen right before your visit.
The best way to avoid sandflies is to stay away from the beach/coastal areas just before and after dusk, or to wear repellent if you do venture out at that time.
Many thanks for your advice. Carlisle Bay suffers the same affliction, fogs regularly, but the hours of dawn & dusk are flea-filled. On the beach itself, however the days are insect free.