Well, yes, there WAS an operating system on the machine before, but I wanted to do a clean install with Win 7 so I got rid of the previous OS by formatting the drive (there was a corrupted system file in my previous Vista installation which could not be repaired, due to a dodgy Seagate external drive).
Now the original thread is perhaps slightly out of date as installation appears to have moved on to the "Setup is preparing your computer for first use" stage. This is also taking AGES - definitely more than an hour on this screen. Installation has now taken 5 hours or so. Is this normal?