but the reRender seems to have not happened late enough
reRender occurs after reques complete and response returned from server. So it could not be later Check a4j:log to check response and updates happened.
Many thanks for your response. I've put the log on and there aren't any errors. I'm against a tight deadline now so I've put a workaround in which shows a modal panel oncomplete and the commandButton in that now does the sorting. This works OK.