Geograph - photograph every grid square

Largely hidden beneath the brambles and forming part of the old airfield defences, a type I don't recognise.
I am now told that this is a control centre for co-ordinating the defence of the airfield rather than a simple pillbox. The structure is largely subterranean, consisting of two separate "rooms" and the observation cupola which is the bit shown in the photograph. This type of fortification was apparently entered via a trap door at ground level but any evidence of such an entrance is now buried under a mound of earth and prickly vegetation.