GEOCACHING ON NATIONAL TRUST PROPERTY IS AT YOUR OWN RISK.
Also, please note that the National Trust is caring for the specialist habitats and landscape you enjoy on this and other caches on NT property, which is one of the reasons we are glad to be members.
Stage 5 - a numbers puzzle
If A = the first day of this year, B was 9 days later, C the day after that, and D, E, F are the same days next year, then the cache is at:
N51 (B-A-1).(D-A)+(E-B) W000 (E-D+2).(F-A) (F-C)
Here is a link to the hints pages, which will help you to solve the puzzle, and which you can click on if you get stuck.