In a similar situation, when I signed up for Sky, my dad referred me so he could get the €75 of M&S vouchers they offered for referring friends and family. They had me signed up very quickly, but as far as those vouchers were concerned, I think they decided my Dad could whistle for them. He gave up after 6 months, but I think I would still be chasing.
Just keep hassling them - you are entitled to recieve the offer they made. Can't you refer them to an Advertising standard body or something?