Hmmm, this seems to have fixed itself overnight. Strange!
As you were...
I recently bought a new Logitech K800 keyboard (wireless, illumiated), paired it to the unified receiver I am already using for my mouse (using solaar), and discovered that the 'd', 'TAB' and left 'ALT' keys were not working.
The keyboard works on the same system when booted into Windows 7, and I can unpair it and connect it via a different unified receiver to my laptop and it works fine.
Both the laptop and this system are running Ubuntu 15.04.
Additionally, if I boot this system into Ubuntu 15.04 from a bootable USB stick the keyboard works fine.
So I'm concluding that there's something left over from earlier upgrades (I can't remember the first version of Ubuntu I installed on this machine!) that's causing the non-functioning key problem. I've tried comparing files in /etc/ with those on the laptop but haven't found anything that changes the behaviour.
Incidently, I have an older logitech wired USB keyboard connected at the same time and it works properly.
Any clues for places to check re old files or configurations would be much appreciated!