Trusteer has fixed the issue that was causing this. You need their latest software update to resolve the issue.
If you uninstalled Trusteer, then reinstalling it (from your bank's original link) should do the trick.
If you just stopped Trusteer, you can do this instead:
Open System Preferences > Trusteer Endpoint Protection
Start Rapport again (top right of the panel).
Click on "Check for updates now" (lower middle of the panel).
Let the spinning thing finish spinning.
Fully close Safari (using Quit).
Restart your Mac (Trusteer needs the computer to be restarted in order to apply updates).
Everything should now work OK.
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Edit:
PS. The link to install / reinstall Trusteer Rapport for Nationwide Building Society customers (UK) is here:
http://www.nationwide.co.uk/internetbanking/onlinesecurity/howwekeepyousafe/trus teer.htm