I’ve finally managed to find out what this strange structure was that I sighted on my camping trip to Dorset back last summer.
Its an Allan-Williams Turret. Its a pre-fabricated structure made out of steel, designed to be operated by a single man. Part of it is recessed in the ground, and the top part can rotate through 360 degrees giving an all-round field of fire. It was designed for weapons such as the Bren, Lewis, Browning and Hotchkiss Machine Guns and the Boys Anti-Tank Rifle.
199 were ordered and built, and they formed part of the many defences rushed into action during the invasion scare in 1940 after the fall of France.
This example was photographed between Dancing Ledge and St Aldhelms Head on the Purbeck Coast.