Seat 1.4 ,51 registered.Why won't it charge?

Our '51 registered Seat 1.4 will not charge the battery,and the ignition light stays on. The fuses are all OK,and there is 12volts on the large cable to the alternator.The alternator has been removed and checked,and is apparently alright.Connections are OK and the wiring looks to be in good condition.On tick-over or reving,lights on or off,no change,no charge.Where do I go from here?