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