If you haven't had a look at the HDAS preview, then do have a look. It's available here: http://alpha.hdas.nice.org.uk/

Some of the features that you can currently use on the preview site are:

Saving strategies Browsing the thesaurus Using thesaurus terms in a search Using limits and fields to refine your search Re-running searches Editing existing search rows Saving results Exporting results Access to full-text links