Deleting a monument_image leaves orphaned records in file_revisions

When a monument_image is deleted, hook_delete is called and should delete related records in file_revisions, along with other records in other tables.

Problem lies with file_revisions. I have created the attached patch to try to fix this but it is not working. It seems that
$result = db_query('SELECT * FROM {files} WHERE nid = %d', $node->nid);
despite the fact that $node->nid is valid and matches what is present in MySQL database prior to deleting.

Can't see where this is failing as yet.