iPhone 6 faulty - always no fault found

Bought this iPhone 6 for my Wife in February. All worked wonderfull, same as my phone, Utopia.<br> April the phone started playing up with very short battery life.<br> <br> I swopped our profiles and I started using her handset and I experience first hand how short the battery life is. I use a phone a lot and can kill the battery in 4 hours max on this handset whilst my own i6 will do the day.<br> <br> Went to iStore where the handset was \restored". This did not help at all and after the second visit"