This patch solves the problems caused by Episode One's upgrade from Source 2006 to 2009 (yes, we know it's 2011 now).

http://www.hylobatidae.org/minerva/filegoatmatic.php?filename=metastasis_patch_2011.zip

If you installed one of the earlier "fixes" which involved dumping a load of Ep1 content into the mod folder, you will need to do a fresh install.