Aha, here we go. The photo was taken in St Mary the Blessed Virgin Church in Eastry, Kent during 1956. The place has a reputation of being haunted, so a Surrey bank manager who was taking pictures of churches for a project made sure this one was empty then snapped away.
The figure is supposed to be a former priest who swore on his deathbed he would never leave (yeah, yeah), and the curious thing is that the layout of the church is different from the one taken in the photo, making the author wonder if the image changed because of the ghost's presence. Time slip caught on camera?
The book this information comes from and which has this as its cover is called Ghosts of Today by Andrew Green, published 1980.