iPhone 5 disconnecting from Wifi

Hey guys, I have an iPhone 5 64gb which I bought about 2 Months ago from London.. I live in Sri Lanka and theres no Apple stores here for me to take the phone to, so whatever help you guys can give me will be VERY MUCH Appreciated!


Here's my problem:


When I opened the box 2 months ago, and switchd it on for the first time it said I can update to iOS 6.0.1 and even before I used the phone I updated it.


And then I had no issues with Wifi..


But then recently when the iOS 6.0.2 update came and I updated it, whenever my phone is in the lock screen it disconnects from Wifi and uses 3G even when theres Wifi, and as soon as I unlock, it connects back onto Wifi about 2 Seconds later..I have noticed that this is eating up my standby battery life like theres no tomorrow..Even when its charging, its connected to 3G and not Wifi in the lock screen :/.. Note, that it automatically connects back onto Wifi after unlocking, I dont need to manually connect.. Its a waste of my battery life because theres Wifi at home and my phone just uses 3G and kills battery during standby for no reason..


Please help me. I need help because this is my first iDevice..



Thanks,

Jamie