I don't know if you will read this post-row post, Phil, but here is a link to give you some background on the wartime structures that we saw on the south side of Baker's island during our "Grand Day Out". Andrew may well have more info on this Starfish" wartime decoy structure if you want to find out more.
http://portsdown-tunnels.org.uk/surface_sites/qsite_p3.html