Sorry was just replying to other comments as well.Have you read post #14?
The current set up is the admin will drag in the file they are processing, the database then opens that file and they can start copying the demographics from the document. If this document is part of a set that has the same (or similar) demographics they can press a button which launches a pop up form that has the details from the last 10 documents entered into the database. The admin then selects the matching record, certain demographics are inserted in the current document record which matches the old document. The current DocID is passed to the pop up form through the openargs parameter of the OpenForms method.
So I know the current DocID (passed in the parameters), I know the Old DocID from which they are copying the data (the admin selects this one using a button) and I know the fields which need to be copied (first name, surname, DOB, suburb etc) but was unsure of the SQL to achieve this. Looks like the SQL method in your answer (and arnelgp's) would achieve the result. Unfortunately the thread diverged into a discussion around database design.
Thank you for your suggestion - I have to do some testing but it looks to be one of the best options.