I needed to add this policy to allow proftpd to start in F-11:
# Proftpd needs to create /var/log/proftpd/controls.log allow ftpd_t xferlog_t:dir { write add_name };
/var/log/proftpd is xferlog_t and it seems ftpd_t can't create new files in directories of that type.
Paul.