Thanks for the prompt replies people! Really appreciate the support...
If I remember correctly, from when I used to have one, it's the number of shots available in the buffer.
Is there a memory card fitted?
Thanks Ken. Yes, there is a memory card inside. I tried my personal memory card which is showing no issues in my camera and we still had the same issue.
...and in addition, have you knocked it to timer mode?
Thanks Yvonne. I'm not sure - I don't think so judging by the icons on the display.
R11 normally just refers to the number of shots remaining (burst iirc), remove the battery and give the terminals on the lens and body a good clean. Also be worth checking you aren't in self timer mode or anything like that!
Thanks Phil. It is in burst mode, so that could have something to do with this... I had already cleaned the body AND lens thoroughly and carefully. As per your advice, I have now cleaned the terminals but no change...
Is the memory card full?
...and how to set up the autofocus options:
http://www.kenrockwell.com/nikon/d80/af-settings.htm
Yes, can't remember whether the D80 has it, but on most Nikons, you can set it that it won't actually take a photo if focus hasn't locked on, so that is also a possibility
...and to reset everything find the two buttons with green dots next to them on the top right. They are the +/- * and AF * buttons. Hold them both down for a few seconds and the top LCD blinks and everything goes back to normal.
Yep, it did that. Annoying at times, but saved me taking OOF shots unintentionally

You can always set it to manual focus.
it would also explain that fact that it works randomly too, its finding focus and firing.
I never had the D80, had the D70s then went to a D200, but between the two, similar settings so assumed it would have it
Thanks guys but I'm not sure this is the issue as when in M mode and on full manual focus, it still wont fire?...
I tried the green dot button thing, Ken - no change either mate.
I'm pulling my hair out here - the last thing I want is to have broken my bosses camera days before he goes on Holiday! >.<
I've taken some pictures of the body in its current state, should it help. (sorry that they're naff - I took them on my phone and seem to have focused on the built in flash!!! >.<)
The lens has aperture rings - I've not seen these before?! Could the issue be anything to do with these?
Thanks again for the support everyone!
Finally - with mention of the battery - it is flashing as low... Will this have anything to do with it?