OK....
Turns out the issue was FastCGi with a low timeout setting. Increased that in IIS and the install finished.
Next, I got a Connection Error which turned out to be an issue with the installed cacert.pem being out of sync. Once that file was replaced everything went smoothly. Well, during the digging for the connection error there was a suggestion that there was a PHP issue so I changed the version from 5.X to 7.1X and ran into another ionCube problem which, thankfully only took a short time to resolve.
Finally I can get on with setting up the system and get it talking to Plesk Onyx, TPP Wholesale etc...