Running xmodmap to set Compose key in /etc/gdm/Xsession doesn't work

[Chris G]

The startup script /etc/gdm/Xsession runs xmodmap with the user's
.Xmodmap file automatically.

I can see it does actually do this because in my .xsession-errors file
I see:-

    Warning: Only changing the first 9 of 32 buttons.

Which is the warning I get when I run xmodmap manually from the
command line.  My .Xmodmap file is:-

    pointer = 1 8 3 4 5 6 7 2 9
    keycode 134=Multi_key

However the "keycode 134=Multi_key" isn't acted on (or at least has no
effect) when xmodmap is run automatically from /etc/gdm/Xsession, if I
run it manually in a terminal window though the Compose key is
correctly set.

Can anyone explain why this might be, and/or suggest how to run
xmodmap automatically at start up so it does set the Compose key?

-- 
Chris Green