UPDATE. The problem has finally been resolved. After more than strenuous demands from Mrs. Scooby that I do NOT go up the tower again I had to resort to enlisting the help of a professional. It transpired that he was able to test the cable from the NVR to what I called "there is what looks like a large bulge in the cable" in #32. It turned out that the "bulge" was in fact a join in the cable and the circuit from the NVR to the RJ45 input to the join was good. All he had to do was replace the join. I didn't think much of the join that he used because it was very similar to the old one i.e. not a waterproof join but he assured me that the black tape that he wrapped it in would keep any moisture out.
When he had done that the camera worked as it always has..
And so, finally, I would like to express my most sincere thanks to all who have been helping me with this pronlem, especially johnfitzy. If any of you want to know how to lay a hedge, shear a sheep, plough a field, or any other farm-type jobs, just ask. Thanks again - Scooby.