According to Ps Bews, it was Pc Myall, but we can't rely on what Ps Bews said in that documentary. The relevant wording was as shown below.
As we go round, Steve Myall says "Oh, hang on, stop. I think I saw someone move." And we look up and think 'Where?' He said "That window up there." And he's indicating as we're looking at the back of the building, top right, so first floor on the right-hand side, no light coming out of the building, and I couldn't see anything. We, we look at it for a while, and I said "Are you sure you saw something move?" And he said "Well, I think so." I said "Well, let's move back." We became quite satisfied, after a couple of minutes of just moving ourselves slightly, that we could replicate the same sense of movement, and it wasn't anybody moving, it was just a trick of, literally, a trick of the light.