Thanks Tony, is there any likelihood that the the OSR deviation would have a carry-over impact onto the NSR, or are they pretty much independent?
To be fair the garage did say that there's always room for readjustment depending on the type of driving I do and how the car is loaded, so they said they'd start with the "standard" settings and that I should keep an eye on it for a month or two and take it back it for a re-check which should show if the uneven wear is still happening (at which point they can re-tweak).
Not sure whether the fact the previous tyres were non-XL MO spec would have impacted their wear, although I guess even if they were wearing more quickly you'd still expect even wear across the tyre.
Possible although the previous owner was a regular at the dealer, and from what I've seen of the car and service history etc didn't seem like the type of person to go with part-worn tyres. There's every chance the issue has been there since the car was first sold but I've only picked it up once the stepping got bad enough to make a significant noise.