Show related invoice no on Credit note

Hi
As per new tax requirements, we are supposed to show the related invoice no on the pdf credit note.

Is there any way to do this? 
So ideally I want to replace the Order no with the invoice no in the below code in reporting/includes/doctext.inc

        case ST_CUSTCREDIT:
            $this->title = _("CREDIT NOTE");
            $this->formData['document_name'] =_("Credit No.");
            $Footer[0] = _("Please quote Credit no. when paying. All amounts stated in") . " - " . $this->formData['curr_code'];

            $aux_info = array(
                _("Customer's Reference") => @$this->formData["customer_ref"],
                _("Sales Person") => get_salesman_name($this->formData['salesman']),
                _("Your PIN no.") => $this->formData['tax_id'],
                _("Our Order No") => $this->formData['order_'],
                _("Due Date") => '',
            );
            break;

Thanks
Sam