Could you let us know what model iPhone you're using, and how old it is? Did you get a new one, or a refurbished one? If it's only few months old, I wouldn't expect it to have decreased battery performance already. In that case, G0ULI's advice will be worth pursueing. Have a chat at the Apple store or give them a call.
Thanks for the replies chaps. I have a 6s Plus that's 6months old and 1 year old by manufacture data. It's more than capable for my needs so far, but this battery drain has been puzzling. I've turned off all the obvious culprits, use geo-location only when using an app. The 'Home and Lock' screen seemed to be using way too much power. I've changed the 'dynamic' screen savers to 'static' and this seems to have had an effect to the positive. I tried to make a 'genius bar' appointment, but it's always a good week ahead as all stores are so full....