To get from start to final find the brass plaque with inscription.
Consider the 5 dates as follows: (for K, JAN=1, etc.)
DEC A. BCDE
APR F. GHIJ
K L. MNPQ
JUL R. STUV
WXYZ
Then the cache is located at:
N 51 R.KL
E 0 D.YIJ
Which should be within 5 minutes walk - otherwise your numbers are wrong.
The information is available 24/7 but the park is closed at night - see the Lee Valley Regional Park website for opening times and also details of wildlife you might see.
Between start and finish you will also be rewarded with good views of the O2 and the Thames.