iPhone 6s Severe Battery Drain Even When Not In Use/Switched Off

I had a new battery put in under the Apple £25 replacement scheme last November. All was fine until a couple of weeks ago when the battery started draining extremely fast. This would happen during the day and even when turned off overnight i.e. I'd go to bed with an 87% charge and it would have 1% or be dead in the morning. The battery health is 93% and I have tried doing a complete factory restore from scratch. All software is up to date with background app refresh off. Any ideas? If the battery says it is 93% healthy, can I assume something is wrong with the phone to cause this?