Type 28 pillbox. The position appears to have been for a field gun, possible 18-pdr or 75-mm but some people have suggested an anti-tank gun.
The structure is stunning and includes a small redoubt to protect the rear of the position. One of the loopholes has 1940 embedded in it.
This would appear to be a bespoke design with the redoubt to the rear providing all round protection. There are also crenelations on the roof to break up the sky line and provide concealment.
Photos from Alistair Graham Kerr (Thanks)