Ok....update time.
I mentioned all your suggestions to my mechanic who worked his way through them...
OBD code - P500 Speed Sensor A.
Have had this previously and he blasted air onto the front wheel sensors and it behaved itself - for a while.
Asked him to clean whatever connectors he stumbled upon - PNP one was clean, but the rear diff sensor was gunky.
Yes, the dash arrow is flashing, as are the lights on the S & E switches.
Once the OBD was cleared he started the car and the main warning light came on immediately - but no other. I drove around doing my usual rural shopping, and once again the up arrow came on and some hesitation in getting out of first gear (which was new). I am starting to wonder if the problem is heat-related, as it often happens when I am about to drive the 50klms home - in other words, after a lot of stop/start shopping.
I took the car back to him before going home, he blasted air on the front wheels and car behaved itself on the run home. (Note-I do live on a 3klm dirt road....hmmmmm)
So, wise ones.....that is the current state. I will keep blasting air if/when the light comes on, and maybe it's all happening because of dust??? Or is there more than 1 issue at play here??
Thanks for looking/reading/thinking !!

[1] Did the person clean the PNP switch, or the PNP harness connector?

[2] With the AW30-40LE or -43LE, there's a speed sensor on the transmission. The ABS speed sensors at the wheels aren't connected to the transmission shift system. The RPM sensor (as it's called) has its own connector on the trans.

[3] If the car wants to start in 2nd, the trans is in limp home mode. You want to avoid driving it in that mode.

[4] Seems to me like the PNP switch itself is cactus.

[5] Try the following - with ignition on but without the car running, rapidly move the auto trans shifter from 'P' to 'L' (or '1') back and forth, about a dozen times. Turn off the ignition, then start the car. If you're lucky, the arrow will stop flashing and so will your mode switch lights; if unlucky, I'd put money on betting your PNP switch has failed.

How far from Berry's or another reliable Volvo specialist are you?

1) Don't know but will ask
2) Aaah, so dusty front sensors is the braking system....hmmmm
3) Amen
4) Seems like that's what's left...
5) Will do when I next see the pesky thing on...
6) Near Port Macquarie, NSW. But I know a good place I can call in Qld.
7) Thank you for your analysis !!

4 days later

A good Volvo specialist repairer thinks it is quite likely to be the Speed Sensor on the tranny.....considering that the OBD code is "Speed Sensor A" then I'll replace it just in case...problem is, when I looked online there are all manner of different shaped units.
Can anyone advise what the one on my S90 looks like or a part number?? I've looked at straight ones, T-shaped ones, L-shaped ones <sigh>
Is this the one??
https://www.fruugoaustralia.com/speed-sensor-for-volvo-10r035882-10r-035882-21660485-22677703-car-auto-accessories/p-200854124-427273470?language=en&ac=google&asc=pmax&gclid=CjwKCAjwnOipBhBQEiwACyGLutSY3rXOYTAw5bljiN5sD8tJNoXBqowRamQU81cTixBxgzy0-pGJnxoC4YYQAvD_BwE

    You will want a genuine speed sensor, if they are available.

    I’ll have a look

    • ML11 replied to this.

      ML11 The speed sensor you want for your AW30-40LE is NLA from Volvo.
      It is p/n 3515641.
      Berry's in Melbourne might have used one, or you may have to go with an alternative.
      See my post in this thread (below) from a while back.
      https://ozvolvo.org/d/16651-16651/7

      • ML11 replied to this.

          Bob is right,

          The sensor is no longer available unfortunately

          Sorry

          13 days later

          One of the problems looking at Forums is that when someone has solved a problem - they happily disappear, and other poor souls further down the line don't get the benefit of whatever the fix was !! So I'll do my best not to do the same, and add to our collective knowledge - hopefully 🙂)
          The story so far.....
          Installed a Speed sensor (LHS of transmission) from Berry MG which removed the fault code, but the engine light remained. Suddenly a new code appeared " ECT problem" ...
          When I researched this - I finally found the answer to the original problem that kick-started this whole rabbit hole I've been down, which is the high speed fan staying on after shutdown, and full-time on while driving. This fellow gave the clues that tied everything together https://www.volvo-forums.com/threads/97-960-coolant-temperature-sensor.38111/
          Now just waiting on a new ECT sensor and hopefully that is the end of the trail?? As an ex computer tech, I know that circuit boards (esp in cars) dry out with time, and we get what we called WIF - Weird Intermittent Failure. A case in point, my previous 740 would start fine but "kangaroo" down the road at no more than 10 kph !! Took a Volvo guy quite a long time to track down the fault - which was fixed by sending the fuel computer away to have the solder tracks re-flowed. Worked like a charm after that!!
          Hmmmm, maybe after 20+ years we ought to remove all computers for re-flowing? Might save a world of hassles? 🙂)

          The ECT failing after 25 years isn't too much of a shocker.
          Be thankful the S90 has only a handful of modules.
          Our 2010 S80 V8 has a total of - wait for it - 57 goddamned modules, which control everything from engine to bum temperature of the seats. It's complexity taken to the ridiculous. The wort part of all is that it appears Ford did the wiring diagrams and they're nowhere near as good as the Green Books for the pre-Folvo stuff used to be.
          Glad to hear you've fixed the problems and how you've reported back. The knowledge base of 960/?90 cars is very small compared with redblock engined cars.

          • ML11 replied to this.

            ML11 CAN bus is the work of the devil, and I leave its care to experts. The V8's wonderful to drive, but I dare not wave a spanner in its general direction.
            I reckon the 960/?90 level of electronics is the correct balance - it does enough to keep things convenient, but not so much as to be a domineering PITA. ABS module, ECU module, and transmission control module is plenty.

              5 days later

              Eureka !!

              The mechanic fitted the new ECT sensor under the thermostat - cleared the codes, and…..nothing !!

              No warning lights, no misbehaviour, no fans staying on when they shouldn't…..just blissful nothing!!

              First time nothing ever felt so good 🙂

              Thanks to all for your good help and taking the time to dispense it.

                ML11
                Excelletnt1
                DAMN IF I DIDNT HAVE THAT HAPPEN TO one of my Whiteblocks , ide forgotten about the ECT....

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