Hi Ian
Is there any additional error information in the server logs?
If not then you can turn on debugging in the config.php file - by uncommenting out the code in the config-dist php file as below and try and install
7. SETTINGS FOR DEVELOPMENT SERVERS - not intended for production use!!!
//=========================================================================
//
// Force a debugging mode regardless the settings in the site administration
// @error_reporting(E_ALL | E_STRICT); // NOT FOR PRODUCTION SERVERS!
// @ini_set('display_errors', '1'); // NOT FOR PRODUCTION SERVERS!
// $CFG->debug = (E_ALL | E_STRICT); // === DEBUG_DEVELOPER - NOT FOR PRODUCTION SERVERS!
// $CFG->debugdisplay = 1; // NOT FOR PRODUCTION SERVERS!
regards