Had exactly the same thing with my Astra. Don't know if you've had it fixed yet, but Vauxhall wanted nearly £400 because it needed the whole hub replaced, not just the sensor. They charded £20 to get it diagnosed. With that information you can take it else where and tell them. The cable had worn due to rubbing on the inside of the wheel. Got a local garage to do it for £185..... Pays to shop around...
Paul
March 2010