Interesting, Robin. Maybe the ratio difference is quite marked, hence the step is (relatively) larger from 4th to 2nd than the others.
In any case, the dealer found no fault, but said they had carried out a basic reset. They also acknowledged that I have raised this issue before the manufacturer warranty expires on Friday. And an MOT passed too.
I'll keep my eye on the box performance. At this time it's too early to say whether it feels any different. It does in fact, but that could just be that I'm expecting it to, based on a reset; the Placebo effect.