First off, I recommend using the ncftp suite for all things ftp via a command line or shell script. Almost all distros have an ncftp package in them somewhere & if they don't, it's freely available and very easy to install from their website.
Do you know the date for which you'll be downloading? In other words, will it always be a file called log.`today's date`.tar.gz? If so, then you can always just get that particular file:
ncftpget -u user -p pass remote-host /local/save/folder /path/on/ftp/server/log.`date +%Y``date +%m``date +%d`.tar.gz
use ncftpls to find the most recent file, then ncftpget to get it...
set file = `ncftpls -x "latr" ftp://host/path | grep tar.gz | tail -n1`
echo "Found $file"
ncftpget remote-host ./ $file