Thanks for the response John Murray.

The $ZV is:

Cache for OpenVMS/IA64 V8.4 (Itanium) 2014.1.3 (Build 775U + Adhoc 14737)  6-JAN
-2015 13:27:05.86

This issue has magically disappeared after about a week.

So we remain in the dark as to the cause but at least it is no longer an issue.

If anyone has any ideas what could have caused the issue, please do feel free to share your thoughts.