For any visitors to
www.trustnet.co.uk who use Firefox and are annoyed by the 'I am a private investor' / survey overlay, here's a way I've (eventually!) worked out to hide it using CSS in Firefox: add the following lines to your userContent.css file in the chrome directory of your profile:
div[class="userTypePopup"]
{ /* trustnet.co.uk overlay */
display: none !important;
}