Bought a car off a dealer advertised as having power

Bought a car off a dealer advertised as having power steering,descovered after 2 weeks of having the car that the power steering didn't work rang them back under my consumer rights telling them about the problem,and they told me to bring it back to sort out.Decicided before I did to take it to a Chevrolet dealers ,paid £57 to find out what was wrong. They told me that the steering rack needed replacing as well as the pump as the rack has caused the pump to pack in.Took it back explained the problem and left it with them.1 week later went to pick up,still didn't work,they had only changed the pump not the cause of the problem the rack,so still wasn't working,asked for my money back.They said no got to accept wear and tear,told me it was working and they would have an independent person to check it, so not happy left car with them and now waiting stuck now with out a car but I was not going to accept it,what do I do now as they refuse to change the rack or give me my money back.