When new, the Kenko tubes would sell for about £90.00. I bought my set for £35.00 having tried many others, these are the best in my opinion.

AF for close macro work is often more of a hindrance than a benefit. I never use AF for macro.

You have a good set of tubes, enjoy them in manual focus.

Welcome to the forum.

Your limitation is not your camera, it's your lens. The Sigma 70-300mm in it's various forms can be quite soft at 300mm. It's not terribly fast either, f5.6 at 300mm if I remember correctly.

I used to use a Sigma 135-400mm which wasn't too bad, but their newer 50-500mm (Bigma) is reckoned to be quite good.

One option you have to extend the range of your existing zoom is a teleconverter, the best of which is probably the SMC Pentax-F 1.7X AF TC. The penalty in using a TC is loss of light, you lose almost 2 stops so your 300mm f5.6 lens would become a 510mm f8 lens which is less than ideal.

I think most members are aware of this deal where they are available at the price you paid, £99.97.

I've had a few A50/1.7 lenses (kit lenses of their time), an F50/1.7, an FA50/1.4, and a K50/1.2. I sold them all for the DA*55mm/1.4 (SDM or MF only, no screw drive AF). No regrets so far.

Of the 50's, the 1.7 is sharpest overall but the 1.4 & 1.2 lenses give you a bit more opportunity in low light, but not much.

With an SR equipped body, a 1.7 is a much cheaper option unless you really want the narrow DOF and superb bokeh of the faster primes.

My edit from above.

That's the kind of compression I was expecting, but it didn't happen. It looked as though it had happened, but on closer inspection, it hadn't.

My machine having spent several hours converting nearly 2000 .PEF's to .DNG's, I've gained a whopping . . . 1.2Gb of space

I'm very surprised , my initial trial suggested I would have saved much more.

EDIT: It seems that Windows 7 is misreporting the sizes of the converted files. I just used the standalone converter to convert 10 .DNG files to .DNG in the hope that they would be compressed.
File sizes of the output files looked to be up to 25% smaller but when I select the 10 original files and select properties, the size of all 10 files amounts to 145Mb. When I performed the same exercise on the 10 apparently smaller files, their size on disk was the same, 145Mb.

25-350mm is an enormous zoom ratio of 14:1 and I personally doubt that they could place such a lens in the DA* bracket.

It's of a variable aperture design too, where all the other DA*'s are constant aperture.

Nobody buys a lens just for one trip . . . do they?

Why not just take the two lenses you have covering the range 18-300mm. The gap between 55 and 70mm is negligible and not really worth worrying about, not just for one trip.

Then again, if it's a once in a lifetime journey, maybe you do need another lens to avoid missing opportunities.

Does that help? . . . I thought not.

I would go for the 18-250mm Pentax lens and take the Siggy 10-20mm and the 100mm Macro lenses as well.

I never go anywhere without my D-FA 100mm Macro lens.

It certainly seems that you've tried all of the sensible alternatives.

Shipping the camera to Pentax should not be a problem, just make sure you use one of the 'signed for' and insured methods with adequate insurance cover for the items you are returning.

Pentax will need the receipt however in order to determine when it was bought and whether a warranty repair or replacement is appropriate.

I can't see why they would need any more than one lens either.

I can't comment on the temperature front because it never gets below about -2C here, and that's in the middle of the night when I have no need of a camera.

Good luck with it and let us know how you get on.