iPhone 4 battery / heat issues

Hi,

For the past week or so, I have been having problems with my iPhone battery life. The back of the phone heats up a LOT, to the left of the "iPhone" text, and I get less than two hours battery life when in standby. Putting it into flight mode works around the problem, but that has obvious problems.

I have not updated the software in the last couple months, and am running iOS 5.0.1. I have read about people having battery issues with iOS 5, but as I updated a while ago, long before the problem started, I cannot see it being related to that. The phone is quite old, and is out of warrenty.

Anyone know what I can do about this?

Thanks,

Callum Macrae