Much of this is now engulfed by the sand dunes, its hard to see where the entrance was or exactly what purpose it served, the roof has also collapsed in on itself and there are a few mangled pieces of metal visible

Petroleum Warfare would have involed the burning of petrol on the surface of the sea to repel invasion.

S number relates to ADS reference ID on Defence of Britain database

The pillbox on Bran Sands has fallen onto the beach due to coastal erosion.

The Defence of Britain database says this as a type 23 pillbox, but I think this must be a variation as type 23 should have the access via a hole in the roof.