Special sort for 1 column

I've just started using DataTables - very nice and useful work.

I have a problem that I can't quite figure out the solution to.
In my table there is one column that requires some special handling when sorting. The column has a class named 'Q'.

I initialise dataTables:

$("#tableLeft").dataTable({
     "aLengthMenu": [[35, 70, -1], [35, 70, "All"]],
     "iDisplayLength":"35",
     "aoColumnDefs": [{ "sSortDataType": "qpos", "aTargets": [ "Q" ] }]                    
});

My expectation is that this will cause plugin's named qpos-asc or qpos-desc will be called as the sort callbacks when the column <th> has the class "Q" and it's clicked.

The purpose of the special sort is to cause any entries that are not numeric or have numeric values < 1 to be sorted to the bottom of the table for an asc sort and the top of the table for a desc sort.

The qpos functions I have are:

$.fn.dataTableExt.afnSortData['qpos-asc'] = function(a,b) {
    var badAssign = 9999;

    if ($.isNumeric(a["queue_position"]) && a["queue_position"] > 0) { aItem = a["queue_position"]; }
   else { aItem = badAssign; }

   if ($.isNumeric(b["queue_position"]) && b["queue_position"] > 0) { bItem = b["queue_position"]; }
  else { bItem = badAssign; }

return ((aItem < bItem) ? -1 : ((aItem > bItem) ?  1 : 0));
                    
};
$.fn.dataTableExt.afnSortData['qpos-desc'] = function(a,b) {
    var badAssign = 9999;

    if ($.isNumeric(a["queue_position"]) && a["queue_position"] > 0) { aItem = a["queue_position"]; }
    else { aItem = badAssign; }

    if ($.isNumeric(b["queue_position"]) && b["queue_position"] > 0) { bItem = b["queue_position"]; }
    else { bItem = badAssign; }

    return ((aItem < bItem) ?  1 : ((aItem > bItem) ? -1 : 0));
};

It appears that the sort functions qpos-asc and qpos-desc are never called.

I'm sure that I've made some irritating newbie error.

Help would be most appreciated

Roy