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. Access was at the opposite end, the near end being bricked up. In this photograph the end wall has fallen away. This could be a defensive structure, with rifle slits - but this would have been pretty uncomfortable with no space between the slit and the roof! Alternatively it may have been an air raid shelter, with the slits providing air circulation? If anyone knows the actual function please let me know.