jQuery Grid Plugin - jqGrid

I have created a basic grid, but am having problems adding a new row in jqgrid - any help much appreciated. I have set up the php file with add/edit as below.

thanks

    <script type="text/javascript">
   
    $(function(){
   
      $("#rota").jqGrid({
        url:'example.php',
        datatype: "json",
     mtype: 'POST',
        colNames:['Id', 'Date', 'HPJ', 'Recycler','CCTV','Assistant'],
        colModel:[
         {name:'id',index:'id', width:200,hidden:false,sortable:false,editable:true},
      {name:'date',index:'date', width:200,sortable:false,editable:true},
         {name:'hpj',index:'hpj', width:200,sortable:false,editable:true},
          {name:'recycler',index:'recycler', width:200,sortable:false,editable:true},
          {name:'cctv',index:'cctv', width:200,sortable:false,editable:true},
       {name:'assistant',index:'assistant', width:200,sortable:false,editable:true}
        ],
     rowNum:30,
        sortname: 'id',
        viewrecords: true,
        sortorder: "asc",
        caption:"Weekend Call Out Rota",
     pager : '#navGrid',
     forceFit : true,
    editurl: 'rotaedit.php?action=save',
    
    });
   
    jQuery("#rota").jqGrid('navGrid','#navGrid',{edit:true,add:true,del:true});
   
  
    });

Here is the rotaedit.php code I am using

    //mysql_set_charset('utf8',$database); mysql_query("SET NAMES 'utf8'");  
     if($_POST['oper']=='add') 
    
    {    
     $id   =   mysql_real_escape_string($_POST['id']);
     $date   =   mysql_real_escape_string($_POST['date']);
     $hpj =   mysql_real_escape_string($_POST['hpj']);
     $recycler    =   mysql_real_escape_string($_POST['recycler']);   
     $cctv  =   mysql_real_escape_string($_POST['cctv']);
     $assistant   =   mysql_real_escape_string($_POST['assistant']);  
     $sql = "UPDATE ".$tablename."
     SET id = '".$id."',
     date = '".$date."',
    hpj = '".$hpj."',
    recycler = '".$recycler."',
     cctv = '".$cctv."' , 
     assistant= '".$assistant."'"; 
   
   
     echo $sql; $result=mysql_query($sql) or die(mysql_error());
     mysql_close($db);    }
    
    
    
     if($_POST['oper']=='edit')
     {    
     $id   =   mysql_real_escape_string($_POST['id']);
     $date   =   mysql_real_escape_string($_POST['date']);
     $hpj =   mysql_real_escape_string($_POST['hpj']);
     $recycler    =   mysql_real_escape_string($_POST['recycler']);   
     $cctv  =   mysql_real_escape_string($_POST['cctv']);
     $assistant   =   mysql_real_escape_string($_POST['assistant']);  
     $sql = "UPDATE ".$tablename."
     SET id = '".$id."',
     date = '".$date."',
    hpj = '".$hpj."',
    recycler = '".$recycler."',
     cctv = '".$cctv."' , 
     assistant= '".$assistant."'";
    
   
   
     echo $sql; $result=mysql_query($sql) or die(mysql_error());
     mysql_close($db);    }
     if($_POST['oper']=='del')  {   }