I have to say that my first reaction to reading this article was that it was complete overkill and that the solution should be to insert into the client table then either join onto that table to do the following inserts (as has been proposed above by admin-499013) or by updating the source data table with the IDs using a join from NT_Client onto NT_SourceData so to still keep it all set based.

This just gets repeated for the following tables, but is only more useful than the approach by admin-499013 if you are either wanting to use the source data again, or if you are after more error checking so each step can be checked as being correct and compared against the source data.

This approach also allows the system to carry on running without you requiring exclusive access to stop other users from upsetting the existing IDs as you are running in the data.

Adam Aspin, can you explain why myself and admin-499013 would be wrong in our approach?

I can see the advantage of your approach compared to a cursor, but it does come with a serious impact upon the users of the system being denied whilst the data is being run-in. This is not a problem with the distinct insert and the re-join approach.