TAKE IT BACK still on the car, USING A MULTI METER PROVE TO THEM that alternator is making 18 volts, (this proves beyond doubt the regulator is FAULTY )your guarantee is more than a day at any motor factor/garage.
norm guarantee for a reconditioned unit is normally 12 MONTHS
DEMAND A EXCHANGE UNIT this is your right they will return the faulty unit to be reworked again
wirecutterman, January 2009