A fix has been found – this looks like a bug in Internet Explorer 8, caused by its auto-complete function.
If you go away to another site other than talkphysics (most important), go into Tools->Internet Options and then click on the Content tab and then Auto Complete->Settings and deselect everything, click on delete auto-complete history. The browser should prompt you to delete all your cache files, cookies, history etc. Check all of them and delete. Then say ok and close the open dialogue boxes. Press Control-F5 (to force the cache to clear) and then go back to http://www.talkphysics.org (make sure you type in the full address using www. as well).
That has (finally…) solved the problem for the users in the IOP offices.