Well that was short lived.
Last weekend there were no faults, no matter how hard I tried to provoke it.
Now we're back to the 'Engine Service Required' when booted and ECM-6805.
It's odd that it can be fine, then re-appear. I expect the turbo was HOT last week after several 'enthusiastic' drives, so maybe the VNT is sticking when cooler.
I'll blank off the engine mounts again to confirm that is (still) not related. There were no vacuum leaks anywhere when I checked last week with a mitty gauge.

May need to give in and remove the turbo to inspect it.

As a(nother) test I blanked off the engine mounts, took it out, waited for the engine to warm, then booted it. 'Engine Service Required' again - so unrelated to mounts (and solenoid).
After a few miles when safe to stop, I turned off and re-started - and no further issues no matter how hard I tried!
I suspect I have sticking VNT in the turbo until it's really hot, then once hot it's OK for a while.

I have tried the '4th gear' trick over the last few weeks, but no improvement.

I assume the next step is to remove the turbo and clean up the VNT mechanism ?

Or has anyone had good results with turbo cleaner fuel additive ?

Initially, you'd be better off testing and watching the VNT mechanism either manually using a vacuum tester https://youtu.be/NxmjRaiqeAk or using the activations feature in Vida.

I did that previously using the Mitty hand-pump - movement seems smooth.

Ok, so use Vida activations now then.
The boost solenoid may not be functioning correctly to divert sufficient vacuum to the actuator.

The VNT does a sweep when the engine is turned on and off so the same smooth action should be viewed however it is tested.
If the VNT is in the incorrect position, you'll also be getting incorrect MAP data at idle and through the rev range.

The solenoid was replaced recently (with a new genuine Volvo part) - no improvement :(

Boost solenoids are easy enough to test and clean without needing replacement and the problem could for example, just be a vacuum leak in the line between the solenoid and the actuator.
Regardless, you don't need to remove and open a turbo to diagnose a sticking VNT so check the MAP reading with ignition on, engine off and at idle and view the VNT mechanism.
Nothing you've said so far suggests a sticking mechanism.

Yes, I did that before replacing it, and while out noticed it leaked a bit (when sucking or blowing through it) but as no improvement and because it's the most common cause of this issue I replaced it.
if there was it would show up on the vacuum tester - none.
Screenshots attached showing the values of various related parameters.
Well, it moves... but if it's moving enough and / or gets stuck at some temperatures and engine speeds I can't tell.

It depends on where you've attached the tester to obtain your reading.

Do the same again but include fuel rail pressure figures and egr position too.

Do a fuel correction test too and post up the results.

The data you've provided still suggests no VNT issue.
Your MAF reading at idle though is variable for some reason.
First reading close to 50kg/h is what it should be. If you check details/description in ECM Parameters, this should confirm.
The 3rd image showing 26kg/h at idle is therefore half of what it should be.
How new and what is the state of the air filter and does the air filter housing look secure with no gaps?

Hi can you also show requested manifold air pressure as well, this will show what the ECU is asking for, so if it matches the actual manifold pressure,
then the vnt is ok, if is a lot different then the vnt is not doing what it should, it maybe the vacuum control not the turbo its self,
you need to drive the car, so vida can log HI Boost,
can you do a graph log with a drive and put the car under load so then you can see if both are close,
Dave