Have now solved this myself. I had made a backup copy of the Dell supplied DVD - we always do this when we only have one physical copy of the media as they so easily get damaged & unreadable.  I was using the copy to install. It suddenly occurred to me whether Dell had implemented some sort of mechanism to prevent using copied DVDs for installation.  I reverted to the original DVD and lo and behold it worked. The fact that the copy didn't crash with any sort of error but just gave a screen with a shutdown button makes me think this is deliberate policy by Dell.  Anyone know if this is true ?
Purchased OptiPlex 3040 with Windows 7 Factory downgrade install.  Now want to install Windows 10 from the Dell supplied OEM DVD. Have followed these instructions  www.dell.com/.../SLN288177 and these  en.community.dell.com/.../19988139 but neither of them work. After changing the BIOS settings & re-booting I get a Dell SupportAssist screen with a white cross inside a red circle and the only option is a shutdown button. Anyone else experienced this and have a solution ?
Problems installing Dell supplied Windows 10 on Factory downgraded Optiplex