Centering a button in a dialog.

[cwr netcom co uk]

Does anyone know the way to center a button in a dialog?
Simply adding the button to an HButtonBox works for ordinary
windows, but dialogs always push the button to the right.
Using GTK_BUTTONBOX_SPREAD makes no difference.

There must be a simple way to do this, but I've rummaged
through the docs and Google for a couple of hours now, and
I can't find a trace of a solution.

Thanks - Will