[CentOS] Help with XDMCP over SSH

I am trying to get XDMCP working over SSH on CentOS 4.4 from a windoze
box running Cygwin/Putty

I have set Enable=true in /etc/X11/gdm/gdm.conf restarted the X server,
logged in with Putty and X11 forwarding enabled. I then try to start the
Xserver with the following commands

# gdmXnestchooser localhost
or
# Xnest :1 -query localhost

Both commands will successfully display a X window and show the X mouse
pointer but not the chooser. Can anyone tell me what I am doing wrong?

Thanks

Dean