Thanks. Blower bulb it is.
Seems to have removed the dust spec, but looks like there's a very slight mark left. Doesn't seem to be showing up in any test shots so far.
Somewhat confused how it got there. I've only used the D80 with the lens I purchased with it apart from trying it with one other lens once a while back. And that's the first time I've so much as looked at mirror lock up. The spec only just seems to have appeared though.
Odd.