One bay north of Bang Saphan and around 70 kilometres south of Prachuap Khiri Khan sits Ban Krut, a cute little Thai town with a pretty and very low-key sweeping stretch of sand lined by a mix of coconut palms and casuarina trees. Think of it as Cha-am without any of the annoying stuff!
Ban Krut is actually just the northern end of the even larger Thong Lang Bay (Ao Thong Lang). As you head south, it becomes Don Samran beach, then Thong Lang beach, after which it hits the headland that separates the area from Bang Saphan.
That said, the Ban Krut area is probably the best of the lot, and when it's not overrun by locals on long weekends and school holidays, it's deserted -- if you like stretches of sand all for yourself, Ban Krut could be for you.
Why stop here? Well, to be honest, aside from some serious tan-work, there's not much to do here other than swim and eat, but that's not to say it isn't worth a visit -- you're on holiday right? There is a cave temple you can visit by motorbike and the hilltop temple offers great views to the south, though you'll need to wander a bit to get a view to the north.
If nothing else, if you're staying in the Bang Saphan Yai area anyway, consider it as a potential daytrip destination, though we'd be inclined to stay in Ban Krut and visit Bang Saphan on a daytrip, but whatever works for you.
Things get a bit livelier (by Ban Krut standards) as you near the Big Buddha towards the northern end of the beach where some good, cheap seafood restaurants are by the beach. Some have English menus -- how's that for flash?
Another advantage of Ban Krut is that it's very convenient to the main north-south train line, so it's easy to get here from Bangkok (or anywhere else on the railway for that matter).