Thanks, v useful advice

Hurray! It has gone. Was a tiny speck, and my makeshift removal attempt was to direct a jet of nitrogen (from a gas preservation system for wine) at it, which seemed to do the trick. Thanks for the advice.

thanks both, useful info. Is there anything to be wary of when approaching the sensor?

Got a bit of a problem. All my images (taken with a k-x) have a sensor smudge in the same place (tried changing lenses, and used dust removal facility). It tends only to be noticeable against a v uniform background, such as the sky.

Examples plus crops below. It is about 1/5 way down the frame and central. What can I do about it?