Just an update.
I have now managed to get the 2.7 OS installed!
Not with Vista though
I put a second hard drive in my PC and installed XP Professional, now the update installs without any problems, same USB lead and port as before.
As I feared it looks like another Vista quirk.
Incidentally now the PC doesn't crash with XP when the Virus is turned off like Vista did.
I think I will now use XP for Cubase and my music production and boot with Vista for games etc.