I realise that I can create a backup of the BCD using EasyBCD. I can restore the BCD from the backup if the system is running, but If I were then to edit the BCD incorrectly such that the PC did not boot, how do I restore the BCD from the backup?
It's unlikely that you'd make your PC unbootable with EasyBCD unless you started playing with the power functions under the advanced tabs without knowing what you were doing or why you were doing it. (It has happened, followed shortly by "EasyBCD broke my PC !"),
Like any power-user tool (regedit e.g.) it is capable of breaking your PC (if for instance you delete all the BCD entries and then try to boot without putting any back), but unlike regedit which just gives you a non-specific catch-all message warning that you could break your OS, EasyBCD will actually give you a specific warning "you have made your PC unbootable, don't try to reboot before you add another entry to the BCD" (or words to that effect). Despite the message (in big scary red letters), people have been known to ignore it ("I didn't think it really meant it"), followed by "EasyBCD broke my PC !" (see above).
I don't quite know the way the mind works for some users, because another scenario which has happened a few times to my knowledge, is users wanting to remove Linux from their boot, perversely deleting the Windows entry from the BCD and leaving only the LInux.
If you should suffer from some such brainstorm and end up where you fear, have a bootable Linux distro burned to CD tucked away in your desk. You can use it to copy the backup BCD over the one you just destroyed.
If your OS is self-installed, you will also have an installation DVD which will repair the boot for you http://neosmart.net/wiki/display/EBCD/Recovering+the+Vista+Bootloader+from+the+DVD.
If you are using W7, and you don't have an installation DVD, you can make yourself a repair CD (Control Panel > Backup & Restore > Create repair disc).
Keep one of those handy before you run into trouble.
Thank you Terry for your helpful reply. I'm using W10 anniv edition. The problem I've got is that W10 consistently fails to update to creators edition (get to 88% complete and then backs it out leaving me with anniv). I want to create an additional partition and clone (using Acronis TrueImage) my current system to create a test OS. I then want to set up a dual boot so that I can boot the test OS whenever I have time to investigate the problem and then revert to the live OS when I have finished. I can add the extra bootable partition using EasyBCD but I don't want to screw up the PC, hence my original question. Thanks for the link to your wiki. W10 allows a W7 repair disc to be made. Can I assume that this would work for W10?
If W10 has the same "create repair disc" facility, then it will be a W10 repair disc. (I have W10 but I never boot it, so I can't check)
If you're just adding a new W10 entry to your current BCD, there's no danger involved at all. Perfectly routine. Adding Entries
Just remember to give the new entry a distinctively different name from the original W10, so that you don't become confused as to which one you want to keep/remove in the future.
Point the new entry to whichever disk letter the original W10 sees the new W10 clone on in Explorer.