As you have the receipt for the works which were carried out I'd go and see the garage that did it and ask them what they'd advise, I find going with a "please can you help me" attitude rather than a "what the hell have you done" one helps.
The worst than can happen is they politely suggest you go away.
Have you spoken to Trading Standards or the CAB about it?
Is it a local garage or part of a chain?