Note to self.  If you are ever going to migrate some Samba shares from using the deprecated SMBFS to using CIFS instead, then instead of just changing the filesystem in FSTAB, then wondering why the dammed thing refuses to mount. Try reading up a bit and realise that the actual mount utility is a different one and is likely not to be installed.

What a numpty.  Anyway 2 tips for anybody whos doing this:

  1. The following is a great way to increase the debug output:
    echo 1 >/proc/fs/cifs/cifsFYI
  2. install mount_cifs