Ok that was odd
first I tried to chvt 1 which gave me an error message- "Couldnt get a file descriptor referring to the console"
Not realising that needed to be run as root (hey it's late) I followed on with a sudo init 3, this caused a brief flurry of disk activity but not much else. I chose to do init 3 because (at least on my old SuSE installation) this should have dumped me on one of the tty's.
Then I tried sudo /etc/init.d/gdm stop which did exactly what it said on the tin and left me sitting at a black screen (I still had video sync) but with the keys still not functioning (therefore I was unable to get to a tty to recover) numlock was still stuck on and caps lock didn't change the caps lock LED.
So then I booted up my laptop and via ssh did a /etc/init.d/gdm start. As I did this I noticed the numlock light flicker and low and behold I was back at the gdm login screen with a fully functional keyboard.
I saw the Nvidia splash screen as I restarted gdm so does that mean that stopping and starting gdm restarts the X server complete ? I am thinking that if xev could report the status of the non functioning keys then it was more likely to be something to do with gnome, but I didn't think the keyboard events were dealt with at that level.
I think I'll put it down to one of those things to not worry about unless it is repeatable.....cheers for the pointers Adam.