The above coordinates are not for the final cache, they will take you to the church itself, where you must collect some information, and calculate the final cache location.
Background:
The name "Blackheath" is popularly but erroneously held to derive from its reputed use as a mass burial ground for victims of the Black Death in the 1340s. Less grisly, but more plausible, suggestions for the origin of the name, which was recorded as early as the 11th century, are that it stems from Old English words meaning "dark soil" (although the soil is not particularly dark except when wet), or that it is a corruption of "bleak heath". The latter seems the most likely derivation.
As an area of open, high ground just outside the City of London, the strategic importance of Blackheath has been recognised since the Romans first built their London to Dover road, Watling Street (now the A2) across it. Since that time, the Heath has played host to more than its share of rebel gatherings, military encampments and exercises, royal meetings, religious festivals, sports, fairs, circuses and a host of other activities.
Onto All Saints’ Church itself: The Earl of Dartmouth, who made the land available, laid the foundation stone in 1857 and on All Saints’ Day the following year (1 November 1858) the Bishop of London came to consecrate the church.
For 10 years the church lay within the diocese of London until 1867, when the boundaries of the ancient diocese of Rochester were redrawn. As south London expanded in the later Victorian period, it was felt that a new diocese was needed to meet the demands of the population. All Saints was added to the new diocese of Southwark in 1905.
Spiralling costs more than doubled the original estimates for Benjamin Ferrey’s church, and it was eventually completed in 1867. It stands on its own on the edge of the Heath and makes no attempt to relate to nearby buildings. It has Kentish rag surfaces and a spire on the south-west corner. Vestries (1890) and porch (1899) were added by A W Blomfield.
Sir Arthur Sullivan and Gustav Holst brought choirs here and the opera composer Alfred Cellier was an organist.
So to the cache:
At the given co-ordinates you will find a black notice board with gold lettering:
On Sundays, Holy Communion is at A:00 am and Evensong is at B:C0pm.
Nearby you will find the foundation stone laid by the Dth Earl of Dartmouth on the E October 1857. There are F vowels on this stone.
At the opposite end of the church, you will find steps leading up to the main entrance. There is a metal railing on the right hand side of these steps with G vertical parts (one appears to be long since missing, don't count this one!)
Cache can be found at:
N 51° 28. (A x D) - B - G
E 000° 00. ((E + F) x B) - (C x G)
Cache is big enough to hold TBs