In sunshine but with a brisk, cool wind, I checked the progress of the boxes. As soon as I got through the threshold, I could see movement around box 20 and the binoculars let me see a pair of blue tits in and out. When I got closer, I could see that there was plenty of nesting material in the box.
As I went around Monks Wood, I tried to see in through the holes, looking for any signs of activity. I was rewarded - boxes 1, 2, 3, 5, 10, 16 and 19 all had visible nesting material inside. So that's eight out of nineteen that may be occupied (box eight has disappeared without trace!)