Repairing a Win8 Installation

I upgraded from 7 to 8 and everything was working nicely.

I came down a couple of days ago to find the PC in a loop, trying to boot into the old W7 installation from the old dual boot screen. I'd forgotten that ages ago I'd installed an early W8 version alongside W7. The OS choices are W8 Developers or W7 - neither work.

I have a W8 installation disc (not the original as I purchased the download version) but the automatic repair option reports failure after half an hour "Attempting Repairs."

Can anyone suggest how I can proceed please?

I would suggest you to start from scratch.. Since you have the disc do a clean install... or you can restore it by system image if you had created one..

Try deleting the old OS's
Windows Boot Manager - Delete a Listed Operating System - Windows 7 Forums

To delete any boot entry (as addition to guide listed above) you can use
Visual BCD Editor.

Just click on entry listed in left pane, right-click and select "delete".

VBCD Editor lists complete BCD contents not just a small part of it as other tools do.