Mike,
All the known surviving structures were researched and recorded on this website
http://archaeologydataservice.ac.uk/archives/view/dob/
Many of the War Diaries in the PRO contain maps showing defensive locations in varying degrees of detail. The problem is that you need to go through all the diaries for every uinit stationed in the area you are researching in the hope one of them kept a map or copy of the defence scheme ie if your interest was Southend it would be no use looking at Essex Regt, but you may end up looking for Scottish or Welsh Battalions that served on the coastal defences. The east and south coast is well represented.
Good hunting. Stephen.