Q: iPhone 6 battery draining rapidly, even after full reset.

I've had this issue for a few weeks now. My iPhone 6 64GB (bought, September(?) 2014 is no longer holding what I would call a normal amount of charge. It can jump for 70% to 60& in a couple of minutes in one go, other times with a couple of minutes of normal usage (messages or just checking the time even) it can drop another 2/3%. Quite often it will sit at say 30% and then just turn itself off and refuse to come on for another 10 minutes or so, eventually it will and then as soon as I unlock the phone it shuts down again. After persisting with it for maybe an hour it will properly come on and then continue to last, however will be closer to 20% by then. It no longer gets to 1% before shutting down, always seems to be around 30/20/10%.

What I have tried:

Home/Sleep button reset

Settings reset

Full content and settings reset

I've restored from an iTunes backup,

I've restored from an iCloud backup

I've tried setting up completely as a new phone and just downloading the apps that I need.

AND STILL the problem persists.

Any help would be greatly appreciated, the next time I can get in at a Genius Bar at a time that works for is next Wednesday...

Thanks so much,

Ryan