LG G 4 battery issues

[Revelations]

I have owned my  LG G 4 since June, intially the battery was fine, most times on average I would get at least 18 hours, I am not a heavy app user's pretty much standard; banking etc

Now here we are updated to Marshmallow, I barely  get around 7 hours, it is getting to the point were I feel just getting rid of the phone,

I have done everything drained the battery took the battery out then charged it, but nothing, I have called o2 support that is comedy, there was no support more like talking to a wall, well it is to expected when you speak with a first level ansewring service,

I want a solution, simple, getting barely 7 hours is not acceptable so I would appreciate any ideas or solutions

[MI5]

I'd try a factory reset but back up first.

[Bambino]

The Marshmallow update is meant to improve battery life. If the suggestion that @MI5 has offered doesn't help, contact LG tech support directly. Your phone is still under warranty.

[Curr946]

I would assume LG have recover tools, try reflashing the firmware & os using the official tools. Or if not available then as suggested contact LG

[Cleoriff]

LG contact details if you need them http://www.lg.com/uk/support