To find this cache you need to find two or more of caches 1, 2, 3 and 4. In each of their log books we've written two numbers that will lead you to this final cache: the bearing from it to this final cache; and the bearing back to it from the final cache. You can use these bearings with a map and GPS to easily find this final cache.
The numbers from two of those caches should be enough for you to find the final cache, all three or four will just confirm the location.
You may find the attached map v3 graphic useful as a guide to where this cache is hidden. It shows 1, 2, 3 and 4 and public rights of way. As you go along, draw a line from 1, 2, 3 and 4 on the first bearing given in each log book. The final cache is hidden where the lines cross. The example below shows the idea, but not the actual answer:
Now walk to that final location, which is beside a public right of way. Most people can then just find the cache by looking around. If you prefer, once there you can use your GPS to read your bearing from here back to 1, 2, 3 and 4. Move around until you match the second bearings given in each log book. This lets you fine tune your position until you are stood on top of the the final cache (well, pretty much!).
All this is subject to usual GPS inaccuracy, of course, so we've also provided an explicit hint - only use that if you're really stuck!
All measurements are averaged GPS readings. I have also plotted everything in my mapping software, and the calculations in that all came out exactly the same.