try using left outer join in the query instead of inner join. for more details on the different join types, see this page:
http://blog.codinghorror.com/a-visual-explanation-of-sql-joins/
On the Enquiries Details Page page, the Add Correspondence link should pass the enquiry_id to the Further Correspondence Form page, for example:
FurtherCorrespondence.php?enquiry_id=3
use a hidden form element to capture the enquiry_id URL variable and store it in the insert behavior.