The cache is not hidden at the published co-ordinates, but it is very close by.
The Church of Jesus Christ of Latter-day Saints, also known as the LDS Church or more commonly the Mormon Church, was formally founded in the United States by Joseph Smith on April 6, 1830. Smith was killed in 1844 and most Mormons then followed Brigham Young to Utah where the Church now has its headquarters - in Salt Lake City.
Embarrassingly, I couldn't actually find out anything about this building in Ruislip, a trawl of the internet did not give any information up and I even had a chat to the Bishop who seemed unclear on any of the early history of this reasonably modern building. If anyone knows any history about the building let me know and I will include it in this blurb.
Once you arrive at the published co-ordinates you should find yourself standing on an inspection cover with various letters and numbers imprinted on a plate on it. To calculate the final Northings, convert the two large letters in the middle of the plate to numbers in the conventional way and then divide the bigger number by the smaller number. Add this answer to the published Northings and you are half way there. For the Westing adjustment, convert the three letters above "08" to numbers, the two large letters in the middle (the ones you used for the Northings) to numbers and the one letter to the left of "125" to a number. Add all these converted numbers up together and subtract the result from the published Westings. You are now ready to find the cache.
If you would like to add to the Church Micro series yourself then please look here
http://churchmicro.co.uk/
There is also a Church Micro Stats & Information page that can be found at
http://www.15ddv.me.uk/geo/cm/index.html