Hi guys

I've had my Xperia Z1 for about a year and a half now and am generally super pleased with this phone.
Up until recently all was good but all of a sudden the battery readings started behaving in a very weird way.

As soon as  I unplug the charger the phone goes from 100% to 95% in seconds.
When I use my phone a fair bit (such as the 45 minute train ride to work) it drops back to about 60% when I get to London.
By the time I get to work the reading is usually in the 50's.
Let's say 2 or 3 hours down the line it would be 30's and any longer than that it drops down to 15% and you get the red blinking LED with the stamina error.

Now the weird thing is that the phone will work for HOURS on the final percentages, it goes very slowly from 15% to 14% etc.
Even when it gets to 2% you can still use the phone for nearly an hour.

To me it seems that the battery readings are completely mismatched with the actual battery life.

Is there any way to return it back to normal?