The search function on this page is not available. As a workaround, go to https://members.prospect.org.uk/ and use the search box at the top of the site. If you're searching for a document id number, put quotes around it, eg "2019/12345"
Browsing these lists
The Collections list shows live documents, but not archived ones. The Document Type list (further down the page) shows all documents, including archived ones.
When you browse by Document type you see archived documents as well as live ones