This is an unusual structure. It is a linear brick lined trench, with a concrete roof and slits on both sides just above ground level. This internal shot shows the brick walls and concrete roof. The openings do not have a separate lintel and are probably unlikely to be rifle slits - the rifleman's head would be uncomfortably next to the roof! Hence the structure is likely to be an air raid shelter with slits for air circulation. If anyone knows the actual function please let me know.