You can contact these people at
http://www.subbrit.org.uk/
They document just about anything underground in Britain. Whilst there is nothing about your particular site on their website they might be able to find an answer. Many underground bunkers that were part of the "auxiliary units" are being reported these days. These people were the stay behind units that in case of German invasion would hide and then attack from the rear. Their bunkers were kept secret, even post war.