Hi David
Sorry to have missed your posting here but, just to confirm what we spoke about on the phone. Anderson Shelters (named after Sir John Anderson, in charge of Air Raid Precautions) were pretty common as the WW2 government encouraged any householder with a garden to build one. As to why your end terrace has such a large plot of land attached, if it’s not mentioned in the property’s deeds then I suspect the reason is lost to history!