Thank you Lawrence Finch (and elcpu) for your knowledge and advice.
I thought I would follow up my post with the story of what has happened since, in the hope that it helps others in the same situation as me.
I tried (repeatedly over a number of weeks) to reset the battery as per your advice. Sorry to say, this did not help. I also installed the Battery Life app...the battery would swing wildly from VERY BAD to AVERAGE, so I didn't know really what it was telling me.
My phone continued to shut down and tell me it was flat anywhere from 9% to 32% level (at one stage, strangely, it managed to get down to 1% charge...where it sat for 16 hours before going flat!) so I contacted Apple directly to tell them of the problems I was having, in the hope of getting a new phone as it is still under warranty.
They wanted me to strip the phone back to Factory settings via iTunes. I did this...the battery problem continued. For 1 week, I was without many of the APPs and features that I use for work and home...until I took the phone to an Apple Authorised Service Provider. I have now been without the phone AT ALL for 3 days...only to now be advised by the Service Provider that "We recently received your iPhone with the issue that the battery turns off at 40%. Apple have recently released a software update (iOS 9.3.3) to combat this issue. I have updated your iPhone for you, make sure you iPhone is up to date with the latest software updates as they could address the issues that you are having, if the problem persists feel free to bring the phone back into us for further testing however if it passes our tests then there may be a diagnostics fee. Your iPhone is ready to collect."
Sooooo...it has been over one month of less than full use of my iPhone 6S as well as 1 week of basic functions only, and then 3 days of no phone at all....only to be told it was an Operating System fault (which, frankly, I find hard to believe...but Apple tells me I have to try the phone out to see if the problem is fixed...again, it becomes my responsibility to sort out a faulty phone that is under full warranty).
I have to say that I am LESS than impressed...angry even...and am left evaluating whether to stick with my iPhone 6S.
PS I can find nothing online suggesting that iOS 9.3.2 was a problem for iPhone 6S batteries. Indeed, Apple would not admit to their being a problem with 9.3.2 when I chatted with them. Do I smell a cover-up? Or, obfuscation?
PPS I note that the Service Provider tells me to always make sure my phone is up to date with the latest software updates...but this would appear to be counter-intuitive as by updating my phone to 9.3.2 supposedly CAUSED the issue in the first place.
PPPS Good times...