That's better!

I don't really know the Metz so maybe this page will help, maybe it won't!
https://www.metz-mecatech.de/en/lighting/firmware-download-flash-units.html

This is the Pentax page:
https://www.metz-mecatech.de/en/lighting/firmware-download-flash-units/mecablitz...
Note it says: 'Now also for Pentax K-1.'

The Firefox plugin FxiF provides that (and actually more that I deleted.)

https://addons.mozilla.org/en-GB/firefox/addon/fxif/

If you're not a Firefox user maybe there's something similar for Chrome or IE.

Consider:

Technoline BL-700
http://www.batterylogic.co.uk/technoline/technoline-BL700.asp

Powerex C-9000
http://www.mahaenergy.co.uk/products/battery-chargers/009-maha-c9000-wizard-one-...

If those are too expensive, the recent incarnation of the Lidl Tronic chargers aren't too bad and can take larger cells too.
https://www.hotukdeals.com/deals/lidl-tronic-universal-battery-charger-1499-avai...
You'd have to wait for these to come round again in their weekly offers.
With this, if cells are heavily discharged it can show them as u/s so it can sometimes help to get them started in a normal slow charger and swap them over after a few minutes. But if the cells have truly gone bad nothing will help. Avoid leaving them completely discharged.

If it helps a bit!

Metz:
Camera Maker: RICOH IMAGING COMPANY, LTD.
Camera Model: PENTAX K-1
Image Date: 2017-10-25 15:54:12 (no TZ)
Focal Length: 77.0mm (35mm equivalent: 77mm)
Aperture: ƒ/5.6
Exposure Time: 0.010 s (1/100)
ISO equiv: 400
Exposure Bias: none
Metering Mode: Center Weight
Exposure: aperture priority (semi-auto)
White Balance: Auto
Flash Fired: Yes (Auto, return light detected)
Orientation: Normal
Color Space: sRGB
Software: PENTAX K-1 Ver. 1.42

Pentax:
Camera Maker: RICOH IMAGING COMPANY, LTD.
Camera Model: PENTAX K-1
Image Date: 2017-10-25 15:49:38 (no TZ)
Focal Length: 77.0mm (35mm equivalent: 77mm)
Aperture: ƒ/5.6
Exposure Time: 0.010 s (1/100)
ISO equiv: 400
Exposure Bias: none
Metering Mode: Center Weight
Exposure: aperture priority (semi-auto)
White Balance: Auto
Flash Fired: Yes (Auto, return light detected)
Orientation: Normal
Color Space: sRGB
Software: PENTAX K-1 Ver. 1.42

That wasn't actually a Ricoh product, Pentax were owned by Hoya then. Recent cameras are much more reliable. Don't base future buying decisions on that camera where they were trying to make it more user friendly having AA batteries at a time Lithium batteries were pretty pricey. As a result Time Magazine listed the Pentax K-x in its Best Travel Gadgets of 2009 list and it won the Technical Image Press Association (TIPA) award for Best DSLR Entry Level in Europe 2010. While your experience is by no means unique others have used and loved them without issue and solved that problem when experienced. It is just a matter of finding the right batteries for it! Surely you can't be surprised Johnsons wanting to charge something to examine an eight year old camera? It must be an hour or two's work to disassemble it, examine it and put it back together then post it back. But if it is just a wrong battery issue there's not a lot they can do.

Here's an updated link to that Youtube video about getting the batteries to work if you followed the link mentioned earlier in this thread and attempted to view it: https://www.youtube.com/watch?v=7LRwoV1Yeac
(Just watched this and it was different to the previous one I'd link to which seems to have gone. Can't help that but sorry anyhow.)

What are you using to charge your batteries? Might they not be getting a full charge now?

Perhaps those are relative scores to other zooms/primes rather than absolute. Also depends on what they mean by sharpness! The Sigma might have one sweet spot that beats the Pentax and they've taken that value. Certainly sharpness won't be consistent over the whole zoom range.

One also has to take into account sample variations, in all probability you haven't got the lens submitted for test and if I was a manufacturer I'd make sure a lens submitted for review was a good'n and not taken at random off the production line. But unless one knows all the facts of the testing it can only be a guide. There's more to lens quality than overall sharpness.

Yes that's very true, any indication as to which is which next to the socket?

Just looked at the camera manual; I see it needs a battery adapter in the battery compartment now. Do you have that?

Do you happen to know the power needed as Maplin do sell multi adapters which I feel sure would work. There's nothing overly special about a power adapter providing it is the right voltage and can supply sufficient amps, and is a power supply and not a charger.

Have you considered taking the camera back to the dealer who said that?