externals update fails if repository copied

I have svn copied a repository folder to a second location, and then
tried to reference some of the (new folder) contents at a fixed revision
in an svn:externals.
svn update does not find the revision, however, an un-revisioned
reference works.
Moving the revision reference to the copy-creation point works, but I
maintain the externals with a script which finds it hard to know at what
revision an object was created. svn info returns the last changed
revision as the ancestor-last-changed, which my script looks at.
Anyone any ideas ?

-- 
David Kitchen
InSync Technology Ltd
13B, Dragon Street
Petersfield
Hampshire GU31 4JN
+44 (0) 1730 235 117