You are looking for a small clip lock box, containing a log book,pen and some swaps..
It is placed close to a path ,which is very popular with dog walkers and other visitors to the Estate,so stealth will be required. It is not in a wall .
Please take great care when replacing the cache, don't drop the cover on the container,and make sure it is not visible when you leave. Thanks.
The coordinates were taken using average waypointing,and should get you very close,despite the tree cover.