Creo que ya lo tengo, he conseguido exportar bien un excel desde mi aplicacion,
Os pongo el codigo:
<?
require('Classes/PHPExcel.php');
include 'Classes/PHPExcel/IOFactory.php';
include 'Classes/PHPExcel/Writer/Excel5.php';
$objPHPExcel = new PHPExcel;
// set syles
$objPHPExcel->getDefaultStyle()->getFont()->setName('Arial');
$objPHPExcel->getDefaultStyle()->getFont()->setSize(10);
$objWriter = new PHPExcel_Writer_Excel5($objPHPExcel);
// writer will create the first sheet for us, let's get it
$objSheet = $objPHPExcel->getActiveSheet();
$objSheet->setTitle('RESULTADOS');
//Informacion de Columnas y Titulos
$objSheet->getCell('A1')->setValue('CENTRO');
$objSheet->getCell('A2')->setValue('MES');
$objSheet->getCell('A3')->setValue('VALOR');
$objSheet->getCell('B6')->setValue('5');
$objSheet->getCell('B7')->setValue('10');
$objSheet->getCell('B8')->setValue(':SUMA(b6:b7)');
//$objWriter->save('pruebaexcel2.xls');
header('Content-Type: application/vnd.ms-excel');
header('Content-Disposition: attachment;filename="pruebaexcel5.xls"');
header('Cache-Control: max-age=0');
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel5');
$objWriter->save('php://output');
?>
Un saludo a todos