The given coordinates should take you to an old sandstone boundary marker for the estate of St Mary Rotherhithe. The boundary marker is a Grade II listed building and was removed from its original location in Stratford High Street and replaced here after the new flats were built.
You will need to collect some information at these coordinates to reveal where the cache is hidden. The year on the Boundary Marker gives you ABCD
On the Boundary Marker the inches gives you F
Now calculate the final cache coordinates:
N51 32.(A-D)(C+F)(C)
W000 00.(C-D)(F-D)(A+B+D)
The final location can be a bit busy at times but the muggles are soon past.
You can check your answers for this multi on GeoChecker.com.