[h-e-w] How to full pathname in modeline

[x_emacs-windows_x]

Hi,

I'm using NTemacs 21.3.1 on WinXP.

I'm trying to get the buffer modeline to display the full file
pathname.

I added this line at the start .emacs, but unfortunately it appears to
have made no difference:

(setq mode-line-buffer-identification (propertized-buffer-identification 
"%20f"))

Now I have not much idea what that does (it was suggested to me as a
solution without any explanation) so there might be a blindingly
obvious mistake...  however I've searched the Emacs manual and
consulted Google and feel none the wiser as a result.

I usually prefer to tweak settings via the customization buffers.  I
did find the Modeline customization buffer, and therein the Mode Line
Format option, but still I'm baffled what to do with it.

Any suggestions, rtfm, or FAQ pointers would be most welcome.

Thanks.

Cheers,
Bernie.