Hi Nick
Look at the properties for any document within your Notes database. Click the meta tab which has <+> as its symbol, and you should see a field called Identifier.
You can copy whatever is in that field, append "?OpenDocument" onto the end of it, and use this as the target for your hyperlink. The Identifier field contains the correct server name and UNIDs for the database, view and document, so you don't have to worry about finding out what they are.
Hope this helps.
Mac