Was drawn to a old book shop today, a couple of pamphlets popped out at me, both published by the OS, covering the descriptions of large and medium scale maps, detailing the old meridians, historical background etc... Not had a study yet, but will make a great basis for some wiki articles.

I suspect there is a third covering small scale maps too

I have a copy of the large-scale one but haven't looked at it for decades.

There is; I believe I have one, somewhere in the pile of boxes that calls itself our second bedroom.