It is advised that you attempt this after 1pm, as that's the time that the general public are able to access many of the gallop fields in and around the Newmarket area. After 1pm you will still need to watch out for any horses that might still be present.
The cache can easily be accessed from the Well Bottom road (B1506) through one of the many purpose-made gaps in the fence. If you approach from the road, once inside the trees you will find a sandy path that runs parallel to the main road, along which you can access the 'Well Bottom' set of caches. Enjoy!