Where do I stand?

[Borgod]

I have an EF XR8 that had a slight misfire under low-rev load. It would miss and backfire a little and usually when it hit around 2000rpm all cylinders would fire and the power would be back. It was driveable, but a little annoying as it mostly happened when it was cold.
It was like this for about a month or two.

I took it in to UltraTune across the road from my work (I can see it from my window here) and they basically told me it was a buggered injector, that I would have to take it back in next tuesday and they would have it for a few days. No worries.

I go to pick it up and when I start it up it was misfiring 100% of the time. Running like absolute $hit, backfiring the whole rev range even at idle, It sounded and fealt like it was running on only 4 cylinders.

I go back into UltraTune and tell them about it, and the mechanic comes back out and tells me that plug No3 was covered in fuel so he replaced it and thats all he did. So he revved it alot (for some reason) and fiddled around with the plugs making sure they were all in and it did SFA. So he basically shrugged his shoulders and said he didn't know why it was doing it. So I had to drive it like that 20km into the city to Ford and leave it with them for a few days (where it still is now).

So basically they gave me the car back 10 times worse.

Because of this I have refused to pay for the 'assessment'.

Talked to him on the phone about it and he said I can be expecting the bill in the mail, otherwise I will be getting the debt collectors.

I still refuse to pay this, and even thinking I will go to court if I have to.
Why should I pay them when they have made the car worse!?

Have I got any chance with this?