They now have FREE downloadable .pdf files of some books I have have already paid a fortune for !
A History of Ordnance Survey
Map Makers to Britain since 1791
The Retriangulation of Great Britain 1935-1962
Retriangulation Diagrams (which I think might be the hard to find Volume 2 of the above book?)
Link is here : -
http://www.ordnancesurvey.co.uk/about/o ... story.html
Some great reading material and plenty of Trig related info 
Maybe not news to you after all - noticed you already had a link in the Wiki pages. Oh well maybe I still pointed somebody out there to them...