To quote from ' A History of Kings and Princes Garth and Forest Hill' by Jad Adams (1993) p42

"The residents of Kings and Princes Garth spent many nights huddled in the basement with the sound of exploding bombs in their ears. Other people from the surrounding area were also welcome to try to sleep in the air raid shelter which was the boiler room later known as flat 9 Princes Garth. A sign on the outside boundary wall facing the station still proclaimed in the 1990s, though in fading script, that a few yards on there is SHELTER FOR 100."

Kings and Princes Garth are the properties opposite Sainsbury's so this appears tp put the shelter on the west side of London Road.