Service @ pitstop

hello there.. I've had such a bad experience at pitstop.. I went to them with the problem I had with my car, they said so and so is wrong with it, I had it done.. the problem was still there, went back and was told to put in two new tyres, after doing that the same problem was still there.. I've taken it back to them today and was told the shaft needs to go in and I have to pay for that. I honestly don't think it's fair on my to be paying for that, it's not my fault they miss diagnosed my car the first and second time round...