If you look at some of the empty plinths, there is evidence of some metalwork, probably lead, about half way up. This would suggest that something has been there, supported at the rear by the metalwork. In addition you will also find empty plinths with new stone or patchwork half way up where the metal support probably was before being removed. I always have taken this as evidence of statues or stonework of some description in situ in the past. Needs conformation by an expert of course.
Thanks to Mike for a great lecture last week.