My vauxhall astra 1.7cdti club model. alternator not charging battery?

i was driving along and suddenly steering became very heavy. car was checked by AA and i was told to replace the alternator. I purchased a second hand one and fitted but still same problem. believing the part was faulty i replaced 2 more brand new alternators and still same problem. since car has been checked and i was told now the ECU (located under the drivers side wing) is not commanding the alternator to charge the battery. Do i need a new ECU? Can you find me one thats fresh to be reprogrammed?
The part number on ecu is 0281011943. its made by Bosch.