Script to summarise data not updating

Question

I have a Google spreadsheet of timesheet data; it has a sheet for each month, each sheet is a lot of six column blocks, one block per client.

I have created a summary sheet that goes and gets the total for each clients and displays it in a list:

function getClientTotals(sheetname, colcount)
{  
  colcount = colcount ? colcount : 6;
  var res;      
  var ss = SpreadsheetApp.openById('myid_goes_here');
  if(ss)
  {
    res = [];
    var totrow = ss.getRange(sheetname + '!A1:ZZ1').getValues()[0];
    for(var i = 0; i < totrow.length; i += colcount)
    {
      res.push([totrow[i], totrow[i + colcount - 1]]);
    }
  }   
  return res;
}

I have then just added a cell to my summary sheet containing =getClientTotals($C$7,$C$8) which passes in the sheet name for the month and the number of columns for each client (in case of "schema" modifications.

This all works fine, however, it does not update when the source data is changed. I have added an onEdit trigger; no joy. It updates if you go to the script editor and hit Save, but that's not useful. Am I missing something?