Spot on Nick, it would have been 57/58. Wall to the barracks was right next to the flats, there was a conveniently placed pile of bricks and the barbed wire had been pulled back so that I could nip over. Believe that the sea end of the wall can just be made out in the picture.
This is where we would often fish for 'dogfish' that inhabited small holes underwater. The bait we used was bread and cheese soaked in seawater and squeezed together, producing a smell that is very evocative to me.