Driving today and every time i slowed down to stop the battery light would slowly get brighter, it would light up completly at idle but go out if i revved it a little but still light up as soon as the revvs dropped. It's a new battery so whats up?

Alternator failing i would say. Struggling to keep the battery voltage, obviously the harder the engine revs the more power you get from the alternator.

 

So at tickover its not producing enough juice so the battery will drain but at higher speeds it will charge... So i expect it will steadily get worse till the stage when it wont charge the battery at all and you'll be left with a car that wont start!