Any ideas?

I've changed the battery as it seemed to make one battery die within a few months of not driving the car. Looked tonight in the dark and the battery light is on, not so much you can see through the day but at night clearly visible. The car is driving like a bit of a pig too and I remember years ago on my old XR2 the alternator made the car drive badly with juddering. Changed it and it was fine.

What's the most likely cause? Belt or do I need an new alternator?

Thanks for any advice