David Newton wrote:
So, it appears that Crown copyright material that has been published in 1954 or earlier is considered public domain worldwide by OPSI and is thus fair game for the Wikipedia to use.
Certainly good news! The copyright laws are rather complex, so I suppose it's not impossible that they could in theory lay a claim to more than 50 years under, say, U.S. copyright law for material published in the U.S. To avoid that possibility, it may be worth archiving this response somewhere, since "the copyright owner explicitly told me I could use it" is a reasonably good defense against copyright-infringement claims, and at the very least would likely severely mitigate any damages that might ever arise.
-Mark