OK So I've had a wonderful conversation with Andy at K-A.
To summarise, what most here probably know....the company Kodak had been using for decades to make 120 backing paper went out of business circa 2015 and Kodak quickly arranged for an alternative supply...which turned out to have problems. Kodak and the supplier worked to fix the issues and after lots of testing they believed they had....but there were still some circumstances where the mottling in particular could happen. The second set of improvements was made in 2017, which by the batch number on my film appears to be a few weeks after mine was made. So I did have defective film. Andy felt that if I'd used it within the expiry date or soon after it would have been OK, but nevertheless offered a replacement pro pack. This is incredibly generous, seeing as I am reporting an issue with expired film. He did advise me that the remaining rolls from my pack would probably have the same problems, so not to use them for anything at all important. He talked about the complexity of manufacturing photographic film, with that being the prime reason it's no longer cheap and why so few companies are doing it now. He did say the only possible way the price might come down is sales increasing....so my personal view is get out there, buy film and shoot!
We went on to have a brief chat about Gold 200 in 120, which he's really thrilled to see come to market. He says they have been able to release a lot of it into the market so hopefully there's enough to go around. He did say that when it's sent for processing, to ensure the person doing scans (assuming scans are being made) is aware it's Gold and not Portra as the profile is very different. The intention is that it is offered at 25% less than the pro colour films.
All in all *very* good customer service. K-A were under no obligation legal or moral to replace my expired film. I don't know if Andy watches these forums, he did mention seeing some users posting 120 Gold pictures on social media and enjoying shooting it. But we're a funny bunch here. Regardless of whether he reads this, I will say that he has delivered fantastic customer service. Not that I've ever had anything against Kodak but this is "above and beyond" and I'm really impressed.