You're on the right path, however in the latest version of richfaces you can't just reRender the modal you have to reRender something in it. Your best bet is to have something like an <s:div> or <a4j:outputPanel> in the modal and reRender that.
Cheers, I thought I had tried that but I'll give it another go.
No joy with that. Will have to keep trying differen things.
Ah, I think I know what the problem is (but not how to fix it).
Because the form isn't being submitted, the backing bean isn't getting populated so clearing it down won't work (it doesn't exist yet).
The form is getting reRendered but not reset.
Looking at this :
http://wiki.apache.org/myfaces/ClearInputComponents it seems like I will have to refresh the entire page.
Post what your new button does, the xhtml and any backing beans it calls.