The redirector is using 'returnredirect' rather than 'ad_returnredirect'.
My foolish attempt to substitute ad_returnredirect resulted in:
"::882: unable to dispatch method 'ad_returnredirect'"
....because it clearly isn't a method!
Am I correct in thinking that 'returnredirect' is a method of the package object? It clearly doesn't accept the -allow_complete_url flag so for the time being I'm stumped again!