Just an update on my lens for anyone who has a 16-85 and faces a similar issue in the future:
I haven't heard anything back from 1stAid yet. Sent the lens to John Pye. (NB for inspection, diagnosis and return of an item if you don't want them to repair it John Pye charge £24 (including VAT and return postage). You don't pay this if you choose to proceed with the repair, which I thought was very fair).
Communication thus far has been very good with them by email, sometimes not so good by phone. Any emails have been answered promptly. Lens diagnosis and quote emailed to me the day after they received the lens.
Diagnosis is that the fault is in the focusing circuit and this needs to be replaced, so parts required. Part ordered, expected to "take a few weeks" to get it. Imagine that it might be a while then........and just keeping my fingers crossed that I don't get an email in a few weeks saying that they can't get the part.
I will update when I have anything else.

Thanks SSF1957

SSF1957, I've mailed 1stAid to see what they say.

Thank you all for the comments.
SMC, as far as I am aware, it can't be converted to screwdrive. It has a DC motor and I don't think it has the necessary mechanicals to convert. I thing some(?) lenses with SDM motors can be converted to screwdrive.
HarisF1 thank you for your insight and for your kind offer. I may take you up on that in due course depending on what route this goes.
SSF1957, I contacted John Pye yesterday and arranged postage but haven't posted yet and haven't agreed anything particular with them. Now in two minds. I came across 1stAid when I was looking for repairers yesterday. How recent is your experience with them? The only reason I ask is parts availability if anything is needed. I am conscious that it seems so difficult to get anything Pentax just now and parts in so many other areas of technology seem to be such a challenge. I had perhaps wrongly assumed that if John Pye were the official repairer they would be first in line for parts. I will give 1stAid a phone and see what they say, before committing to anything.

Thank you. I will have a look.
Yes, I have loved the lens, until now. My favourite and most used lens.
I've had it for over 4 years, so well out of warranty, so looks like an expensive time coming up.

Thank you. Anyone had any recent experience with John Pye Ltd?

Sorry, discovered that John Pye is the official UK repairer.

Hi, my 16-85 has suddenly developed autofocus issues. It has always focused really well but has suddenly started to completely fail to focus at most focal lengths both through viewfinder and live view. It seems to focus fine at 85mm and it will still focus occasionally at 16mm but will not focus most of the time at other focal lengths. The lens has never been dropped and i do not recall bumping it at any time. I have it on my K70, cleaned all contacts on both lens and camera but no change. I also tried it on my old Kr and it exhibited the same issues there. I can still focus it manually at all focus lengths. Anyone have any suggestions on;
-what is wrong with it
-If it needs sent for repair - where is best in UK