the exception was on line 313 of ExistingRelatedRecordForm.php
There is a type in the variable name$this->_relationshipname it should have a capital N for Name
if ( !$returnPage ){
$returnPage = $app->url('-action=related_records_list&-relationship='.$this->_relationshipName);
}
He had an odd setting overridden in Firefox resulting in this referrer path not being sent to the server
eg about:config network.http.sendRefererHeader was set to 0
fixing that back to 2 got him going down the normal path again..
But it uncovered this bug
Cheers
Richard