[Commons-l] GFDL vs GFDL-en in derivative works

[Nilfanion]

I ran across a typical badly-attributed derivative of Wikimedian
images and corrected the page
(http://en.wikipedia.org/wiki/Image:UK_Legislatures.png). However, one
of the source images is GFDL/cc-by-sa multilicensed and another is
GFDL-en licensed only. Is the GFDL compatible with GFDL-en for the
purposes of derivative works - and if so should the combined license
be GFDL or GFDL-en (preserving those disclaimers)?

Unfortunately, the author of the GFDL-en licensed work has gone, but
the multi-licensed picture's author is active - so the image could be
preserved by getting permission for GFDL-en. That's sub-optimal, but
it still beats deleting....