Hi @Ash69. I'm sorry to hear that you are facing this problems after the latest update. Although from my point of view Oreo has its drawbacks, your phone should work fine with it...
On the one hand, battery life depends on the use you do with your phone, so if you're playing a lot of games and making a lot of use of the screen, 15 hours is not so bad. However, if you state this complain, maybe you are feeling that the battery life is lower than before...
Anyway, after a major software update like Oreo update, it is advisable to perform a software repair using Xperia Companion. I'd recommend you to try your phone after that without restoring your apps at the beginning to discard any compatibility issues. Also take into account that after a software repair it will take about a week to recalibrate the battery, so the battery life cannot be reliable in that period.
Hope this helps. Please report back if this solve your problems.
