200tdi battery staying on?

Hi ya all,
I have just become the proud owner of a disco 1 which the battery light stays on pretty much all the time.The battery is still charging ok I have been told it is a black resistor thats on the back of the alternator is this true?
If so wheres the best places to get one or is it main dealer only?

cheers

it's a diode not a resistor - before you start taking things apart check the belt isnt loose first

I rang land rover today and its a capacitor they reakon and cost all of 10quid plus that lovely vat.just got towait now for it to get here.