Due to distances to existing caches in the area, I wasn't able to place it at the milestone, so the final cache is not located at the above co-ordinates. However, you will need to visit in order to collect some numbers written on the Milestone:
Distance to London = AD
Distance to Chelmsford = I
Distance to Brentwood = H
Please do not post spoiler pictures of the milepost in your logs!
You will also need to collect some information from the following Are We Nearly There Yet? caches:
AWNTY? Mountnessing
AWNTY? Heybridge
AWNTY? Church Road
The final cache is at:
N 51° F(H-D-A).DB(C-E+G)
E 0° AD.(I-H)(A+D)(I-F)
You are looking for a magnetic nano a short walk from the milestone.