I'd like to read that John so which Pentax instructions are you referring to please? I've been unable to find mention in any of the Pentax DSLR manuals. I've read so many times that Pentax cameras don’t actually need a circular polarizer to operate whereas other makes do require it.
One thing to note is that not all polarizers are born equal and there are some poor ones around.

Are you certain that applies to all Digital Cameras? I've read elsewhere otherwise with regard to Pentax, indeed I do sometimes use a linear one (as I've had it donkeys years) in preference to wasting money on an unnecessary new one.
All I can say is if you're buying new get a circular as it will generally be more compatible for use with other brands, but if you've inherited a linear don't dismiss trying it before binning it unnecessarily.
All I can say is I've not experienced any issues with the old linears myself. Have you Richard experienced an issue first hand or are you repeating something read off A N other brand web site?
I wonder if this isn't just a myth by manufacturers to sell more filters!

Not even polarizing to reduce unwanted reflections?
Not even ND to allow a longer exposure?
Not even a yellow filter with panchromatic film to enhance the sky?
Filters can have their uses.
Some Pentax lenses actually have filters built into them, you can't not use them with those! That includes a Pentax lens first released within the last seven years (DA 25mm f/4)

I think most here will have read the reviews and know what the lens is (trust you got copyright clearance from WDC to reproduce their article?)
What you haven't said is perhaps more important, how much are you looking to sell it for?

My advice would be to wear a hat or risk having a head gouged by the nesting Arctic Terns.
Great photos by the way.

That tube would certainly help but it would be better if you could find a set of three with different sizes. Just a single tube would be somewhat limiting. At least that one has all the electrical contacts and aperture lever so it's worth considering.

They're good Paul. Good sharpness and neutral backgrounds.

Last week there was a photo of Algi uploaded, now you've spotted LennyBloke out and about!

They all look good to me Peter. Even the scans have transferred well.

Yes, there are a lot of variables!

As to uploading to PU a jpeg straight out of camera; that is usually a large file to upload. It's best to resize it to 1000 pixels on the longest side after any cropping and other processing including making sure its the right way up. If the photo isn't resized before upload, the site will have a go at doing it for you, but can on rare occasion fail to succeed with that.

I expect they were put off by the reception the K-01 got after getting a top designer to create the updated box camera based on the K-5 internals.