Perhaps the seller could explain a little more fully before we jump to conclusions.

I'd go for a second Pentax battery, others will opine that some cheaper ones are just as good.

Yes, I missed that bit.

In that case David it looks as though the addition of a polariser has upset the optical configuration. I've never really used polarisers on very long lenses but I seem to recall that they are particularly suceptible to image degradation from filters. Perhaps try another polariser to see if that shows the same behaviour?

Because it loses two or three stops of light, the maximum aperture of the lens at longer focal lengths no longer enables the use of AF.

I'm not sure why you feel the need for a polariser on such a long lens?

Do you have the camera set to AF-C? This setting will cause it to continually focus.

There's got to be a reason for this. We have two K-5s and neither shows any tendency to gather dust, so where does the difference lie?

The only thing I can suggest from the scant information given is that maybe something in the lens is being drawn into the camera body when zooming. Or maybe there's some manufacturing debris in the body that keeps settling on the sensor. There shouldn't be, but there must be something going on to cause this.

Cosseted in its cosy world of bubble-wrap, the lens arrived today, safe and sound. It stretched, no doubt weary from its long journey, unzipped its coat and now resides happily on my shelf waiting for its first walk.

Looks good and I'll look forward to trying it out soon!

Excellent, we'll look forward to seeing some shots soon!

There seems to be a lot of mention of diffraction just lately. Diffraction sets in at smaller apertures, progressively reducing the resolution of the image. However, don't assume this is a major issue. With a good lens excellent results will be obtained down probably to f16 and f22 should be used if DOF is the main requirement. Yes, some sharpness will be lost, but it should still be good. It's good to have the choice.

With tiny sensors, they are often limted to f8 as beyond that quality drops off, but also because there's plenty of DOF anyway.