Ok, there's good news and bad news.
First the bad news - it happened again this morning
Now the good news - it's MUCH MUCH more subdued in nature than before! I had all the symptoms again, raised idle, excess noise and slight stuttering but frankly they were around 20-30% of what I usually get. Usually when it happens it's the noise I hear first, cutting through the stereo sound unless I have that really loud. This time the first thing I noticed was the raised idle when I came to a halt - couldn't hear any noise above the radio. Turned the radio off and drove for a bit and, sure enough, it was in "tractor" mode but didn't sound like a tractor!
Don't get me wrong, the symptoms are still there but they're just very very reduced, to the point where I could happily live with it. Driving along normally with the radio on at a sensible volume, you'd struggle to hear the slight increase in noise tbh.
I'm now praying that this was a result of the work the dealer have just done and not just an amazing coincidence that it picked this time to be more subdued than ever before. If the next time is similar to this then I'll have real cause for optimism that the problem has been (largely) cured.
Spoke to the dealer again today and apparently the software update has nothing specifically to do with this problem, it was available so they thought they might as well install it. Apparently the update dates back to 28th May this year! Similarly with the exhaust gas temperature sender, this was supposed to be check and replaced on vehicles which produced a certain type of fault, again not the one I (and you lot) have been having, but they thought they'd replace it anyway.
Despite this it would appear (pending confirmation from future occurrences) that one of t'other of these modifications has caused this change in behaviour.
I'll keep you all posted on any more developments