Thermal store valve-thing leaking?

Hello, we have a thermal store heating system and attached the the water tank is a silver ball thing...
http://img4013.photobox.co.uk/788959023e57e5d115e97f207858c43f591a59082e8e18b603df98880c05094acb7212ac.jpg
The ball thing is leaking around the middle, as you can tell. We have no idea what this is and therefore have no idea if we can replace it. Help would be greatly appreciated. Thanks