Apple replaced my iPhone 6s battery - for free!
For quite a while now my iPhone 6s has been shutting down despite the battery meter registering over 30%. I had assumed it was a calibration error on the meter but when the phone recently shut down with over 40% charge remaining, I decided it was time to search the internet to see if this was a known problem. I discovered that there had in fact been a small number of iPhone 6s devices manufactured in 2015 that had faulty batteries, and mine was one of them. This was established by entering and submitting the serial number to Apple.
I chose to go to my nearest Apple Shop in Cambridge (UK) but when I selected this option on the web page, it offered an appointment the following day with no option to change it. This wasn't convenient, so I phoned the store. After negotiating the automated menu, I think I ended up with a central technical department, not the store. Having explained the issue, the technician ran a remote analytics on the phone and confirmed a battery fault. He made an appointment for me to visit the store at 15.00 on the following Saturday.
The store was frenetic, deluged with people wanting the new iPhone X. I had arrived 20 minutes before the appointment and was told to come back 5 minutes before. It seems that the appointment system is very precise, although not being prepared to take the phone a bit earlier seemed somewhat pedantic. Anyway, I returned at the suggested time and the job was 'set up', after the in-store technician had re-run the analytics and confirmed the battery fault. Just under two hours later I returned to collect my phone with its new battery, free of charge.
The overall process was a bit bureaucratic but with a bit of patience I got there in the end, and hope now to enjoy a few more years service from my iPhone 6s.