Silent installation with existing mysql server (no replies)

I am trying to create a silent installation of MySQL 5.5 but my problem is that it will be installed on PCs that may already have an existing mysql server for which I dont know the root password.

So I thought I could set the install path instance name and port number to something different and create a second instance. This approach works fine with a normal attended installation but just brings up the installation dialog (not silently!) if I do it from a batchfile

This is the batchfile I am using
(adapted from http://forums.mysql.com/read.php?11,277117,277165#msg-277165 )

rem echo off
cls
echo Starting Install...

set mysql_msi="C:\dtdata\mysql-installer-5.5.15.0.msi"
set mysql_svname=MySQLDT
set mysql_port=3308
set mysql_datadir="C:\dtdata\database"
set mysql_cmd="GRANT ALL PRIVILEGES ON *.* TO 'root'@'%' IDENTIFIED BY 'mysql' WITH GRANT OPTION;"

msiexec /i %mysql_msi% /qn INSTALLDIR="C:\dtdata\MySQL\" /L* C:\dtdata\MSI-MySQL-Log.txt
echo MySQL Version 5.5.15.0 installed...

"C:\dtdata\MySQL\mysqlinstanceconfig.exe" -i -q ServiceName=MySQLDT RootPassword=mysql ServerType=DEVELOPMENT DatabaseType=MYISAM Port=3308 RootCurrentPassword=mysql

echo MySQL Instance Configured...Service started...

This post seems to be about the same issue but there were no replies:
http://forums.mysql.com/read.php?11,406919,406919

Can someone advise please?

Thanks

Peter