Clause 23 is only applicable if the Contractor is an association, with the members being the Third Parties to it. Please see:
http://www.finance-helpdesk.org/front/ShowArticle.aspx?ItemID=568. In this case, each third party would have to submit their own Form Cs and Audit Certificates.
If you are not an association, you can still have a Third Party, if it is approved in the DoW. In this case, The Third party costs are input into the contractors Form C under the Third Parties Column.