Amazon have passed my issue onto their advanced technical team. I'm hoping they can find out what the problem is or maybe roll back the update on my kindle (I tried to do this using a previous .bin file but it didn't work - no surprises there).
If they can't find anything wrong with it over the next couple of days they will offer me a replacement.
Although I have had multiple problems with my 7" Oasis, I still think amazon are wonderful to deal with wrt customer service (I always use their chat function, by the way, I know some of you have had problems dealing with them over the phone).
So I'm really as happy as I could be about this right now, glad they have taken this seriously and are willing to investigate the software issue while making it clear I will definitely get a replacement if they can't find a reason for the issue.
So, thank you to the person complaining about people complaining about battery life, you shook me out of my complacency and made me realize my battery life was completely unacceptable - the very opposite of your desired outcome.
And, of course, a
sincere thank you to Harry and others who have been helpful and supportive.