In case anyone else gets this.. this was due to ImageInvoker not finding the DS installation path in their setup. Kamino tried to work around this with a silent install.
I resolved this by putting in the DS string registry value InstallDir in HKLM\SOFTWARE\Altiris\eXpress which points to the express share. Then I was able to install the new version.
We then had issues with the service not working and the logs cited SQL errors. This was because his SQL collation was case sensitive and I had some queries in the code which were not case matched. So, I corrected those case issues and compiled a revised MSI (version 1.0.7) to fix. This is currently the version attached to the ImageInvoker v1 download.
Thanks to Kamino for being patient whilst I worked through this one!