I did the uninstall old drivers first, rebooted, then install, rebooted.
Here is a link to driver sweeper. This may help.
Strangely, when I reboot, an ATI driver (8.501.0.0) is present and I can see the display normally. ATI Catalyst Manager isn't (of course because I haven't been able to reinstall it). If the software is not seeing the hardware then that's a pretty dire situation.
I have a load of work to do now but when it's done I'll reformat and reinstall - looks like the only answer.
I have had to do that just to change from ATI to nVidia.