root password is unrecognised

[samhain]

What language / kb layout are you using?

Good catch!

British English

When I thought about that I realised that the password had a character in it that is in a different position on British and US keyboards so I:
1. Set a new password that doesn't have that character in it (using sudo su - I had added my user to sudoers using a separate boot) and that worked.
2. Then again using a root terminal I changed it back again to the original one. And to my surprise that now works also.

So I think it is possible the installer in 8 may have some confusion about the keyboard layout. After all I have been through a number of installs of it that have had the same result with the same password. I do specify British English in the installer FWIW.
Maybe I am the only person who uses such a character in passwords && has British Keyboard && installs Debian 8 from the live DVD

For some reason, resetting the password via grub did not work.

But changing it in a root terminal eventually did.

Anyway it now works, and there is even an hypothesis to explain what happened.

So thank you

[samhain]

3) If you're using the same medium (or the same .iso) to install now as you used a year ago, it's certainly possible that either your .iso or your medium is corrupt. Have you verified the checksum on both the downloaded .iso file and the medium you're using? (At the risk of belaboring a point, verifying one without verifying the other is a waste of effort.)

For the record I verified the iso against the sha256 hash, and used cmp to compare the DVD and the iso (returned nothing)

The iso I used was: Debian Live 8.7.1 amd64 xfce, which I downloaded and burned to disk on Wednesday last week, so it was not the one I had used before.