TrigpointingUK

You might want to try adding a new top similar to what you see in page 178 from "the history of the re-triangulation of GB 1936-65". You can download the entire document from the OS web site. The page shows one that the National Trust/OS made. I have no idea who would make the new top for you.

Interesting (and good) idea.
I must have looked at thousands of BM's over the years though...