Thanks all..
To clarify a few points, and to avert the disagreements...
First up, I've had it in 6th doing 60km/h within 500m of leaving home from a cold start. The occasions when it's played up have been when warm (while not at temperature, certainly showing around 70-80 degrees).
Second, the change point when downchanging is around 3000rpm. 40km/h in 3rd is roughly 1400rpm, so 2800rpm at 80, which would appear to be well within limits for a 4500rpm rev range.
What's happened, only a few times, is that the gear change points seem to go all over the place - auto changes suddenly happening at 3000-3200rpm, instead of 2000-2500rpm for similar throtlle settings, the box not responding to reasonable (if unusual) gear selections, either upchanging or downchanging.
It's happened both from the paddles and the tiptronic part of the gearbox.
Probably time to go back to the dealer and ask them about it... the software may have corrupted somewhere.. but also keep a bit of an eye on it again and try to be more observant..